Speaker wants 100 watt RMS. Is 40 watt tube OK?

I’m considering changing form parasound a21 to Manley Mahi monoblocks (40 watt tube) to go with my Monitor Audio gold 300s.

the speakers (90 dB sensitivity) say they want minimum 100 wpc.  

What kind of problems (any?) will I run into?
90 dB sensitivity, 8 ohms, you should have no problem as long as you don't have a huge room.
otherones, depends on what you like listening to and how loud. If you are a stadium  rocker forget it. Jazz, classical and light rock at moderate volumes no problem. Personally, I think you are making a mistake. Save a bit longer and get a more powerful tube amp if you have to have a tube amp. 

90db efficiency only tells one part of the story.  How the speaker’s impedance changes over the entire frequency range is very important for tube amp matching. 

I have PrimaLuna mono-blocks that put out 70 watts per side, and 92 DB speakers in a moderate room, and I'm quite satisfied.

They can play much louder than I want them to. I like jazz, as opposed to rock, if that makes a difference.

No matter what anyone says, 40 watt tube approximates 100 watt SS. The size of the room is the dominant factor in this equation.
Absolutely agree with testpilot's comment and actually more information is needed to even make an educated guess as to whether you might have problems, room size, music, listening volume, and more speaker info. While searching for MA Gold 300, two different speakers show up on the MA website, one being discontinued. The current Gold 300 is a 4 ohm speaker and will likely measure 87db like the PL300, also rated at 90db, did in a review. Since MA recommends a minimum of 100w on either of the Gold 300's leads me to believe there is something going on with the impedance curve resulting in a speaker that would not be friendly with lower power tubes.

tls49 is correct in his analysis, but I suspect in a small room at non-head banging volumes the combo will work fairly well.  Tell tale signs would be harshness on peak signals -- think loud cymbals or orchestral crescendoes.  It will be subtle, but easily heard if present.
No matter what anyone says, 40 watt tube approximates 100 watt SS. The size of the room is the dominant factor in this equation.
Is there some law of physics I missed in school?  This may be sorta true in some cases, with some amps but it's far from a universal conversion factor.  I can think of many circumstances with my own gear collection where this is absolutely not true.

For instance, what about damping factor?  While I'm not a nut on the subject I will say that a 40W tube amp w/ a DF of 3 will not sound anywhere near the same as a 100W SS amp Class A/AB w/ DF of 1,000.  Speaker Q is fixed by the design and the the same speaker will be driven very differently with amps of varying DF.  That is not to say that both amps may not sound good (which is the decision of the listener) but they definitely will not sound the same.

I had a 150 watt SS amp in my listening room, and now I have 70 watt mono-blocks; when it comes to volume, there is no difference.

Many people have said the same thing; I earned a living as an electronics technician, so please believe me, I know all the formulas.

BTW, they definitely do not sound the same; the tube mono-blocks sound much better.
The Manley’s use EL84 which are not exactly big power tubes. In fact, they run in triode mode at 20watts (UL 40w) which will sound more nuanced and you may prefer it that way which limits you to a fairly low power set-up. If you do like to rock out, you may prefer a KT88/150 based amp.
The difference between 150w and 70w is ~3db. This is a very slight difference in volume. The a21 amp is 250w, so dropping to 40w is a difference of ~8db, and if the OP's Gold 300 are the newer 4 ohm model, the a21 is 400w, then making it a 10db difference. These larger drops in power have the potential to make a much more noticeable difference in volume.

With over 25 years experience working in the audio industry, I have been exposed to many more amp/speaker combinations than most people. A common problem has always been underpowering a speaker, not so much for volume, but for speaker control. This is likely why MA recommends a 100w minimum. So, what kind of problem? Depending on the music, a significant loss of dynamic range could be the result. Why not contact MA for their opinion?

According to Benchmark Media FAQ distance from the speakers matters only at the open space. Inside of medium size room we can assume that 1W (2.83V@8ohm) will produce 90dB loudness with 90dB/W speakers. Each 10-fold increase in power will result in 10dB loudness increase (twice as loud). 100W amp with 90dB/W efficient speakers should produce 110dB. Do you need 110dB? Do your neighbors need it?
100W is 30% louder than 40W.

(Perceived loudness = k^(1/3.5), where k is ratio of power)
@kijanki But that’s the theory. The reality is that all things being equal, more power can bring an authority to the music like a firmer grip and more breathing space for the music dynamics to fill. It is also true, that much of the action, at medium volume, takes place in the first watt. It all depends on whether the OP wants to crank it.
@mijostyn +1.  It may sound OK at low levels, but you will have no headroom, smaller soundstage, weak bass among other issues.  Not having the headroom is the biggest issue for me.  Also, the idea that tube watts are somehow more than SS watts is pure nonsense.  
Forgive my engineering ignorance....but isn’t there also the consideration of how much wattage is actually utilized on average? I’ve generally thought enormous power “on demand” may not necessarily guarantee a firmer grip on the signal - or be needed - if the circuitry of the amp can allow it to peak and sustain well, and your speakers are rated for a decent relative efficiency. Btw, I’m not asserting this idea, more asking here! 
If the loudspeakers are 87db sensitive then the Manley amp at 40 wpc should be able to generate roughly 101dB at the listener position.  If the average listening level is 80dB, which is loud, then there is 21dB of headroom.  That's a lot of headroom.  Even if the amp is run in triode mode outputting 14 wpc the listener still has 16dB of headroom.

The Monitor Audio loudspeaker is a 4 ohm load, but it's not really hard to drive.

@riccitone, what is delivered to speakers on average is in order of few percent on peak power.  It is because assuming half of the loudness (on average) means 10% of power, not to mention that music has gaps.  In addition power specification is very vague.  It is possible to find amplifier that can deliver huge peaks but is rated for lower power because of heat dissipation.  Such amplifier might sound much louder than, for instance, class A amp with smal headroom.  It also depends on the type of music you listen to.  Jazz trio music will need very little average power while heave orchestral pieces might need much more.

@noromance, true but it might be related to quality and type of the amplifier and not the power.  People might replace low power amp with better quality higher power amps or replace tube amp with SS.

@br3098  Damping Factor is pretty much useless for the "damping" alone.  Theoretically the highest real damping factor you can obtain is about DF=1.5    When speaker membrane moves on its own it produces back EMF (of the same polarity that would otherwise cause this motion) but current direction is the opposite - from the speaker to the amp, causing opposing force on the membrane that stops it.  This current circuit contains speaker, speaker cable and amplifier.  Speaker, that is in the circuit, has about 2/3 of its impedance resistive.  For 8 ohm speaker it will be around 6 ohm at low frequencies.  There are amps that sound great with DF=1, but if you don't want to make it worse (for the purpose of damping) DF>20 should be sufficient.  

Great explanation, thank you! 
And in my case, definitely acoustic and jazz through very efficient speakers and a SS amp (considering some tube monoblocks!)
That idea that 3 dB is a very slight change in volume is ludicrous. 1 dB is a slight change. Raise one channel 3 dB and the other channel will disappear. The image will shift all the way to the louder side. 
Damping factor only matters at very low frequencies below 100 Hz.
Back EMF is highly overrated. Yes the speaker is a generator but a weak one. Only very large subwoofers with huge magnets might present a problem but the SS amp is protected by it's output stage's very low impedance and the tube amp by its transformer.
Power is power. Tube amps my clip more gracefully but any clipping is distortion. 
At any rate subwoofers require a high power amp with a high damping factor (low output stage impedance) Above you can pick your poison and power is not as critical. 
Agree with @mijostyn but wish to add one point. Blown speakers are almost always the result of insufficient power. When an amp is driven into distortion it causes stress on the drivers leading them to overheat and in some cases fail. It's true that tube amps clip more gracefully but I would err on the side of more power. Monitor Audio probably has a good reason to specify 100 watts of power.
I have seen speakers being driven with too much power, as well, causing destruction to the drivers, and sometimes, crossovers. Lets keep in mind, that the volume control setting on a preamp, does not determine the wattage used, of a connected amp. Many factors in determining proper power. Speakers being used ( size, relative to room, sensitivity, and impedance ); room size ( distance between listener and the speakers ); specific music being listened to ( a single cello and violin duo, or a full, grand orchestra, consisting of canon shots, just as examples ); maximum volume desired by listener; room acoustics ( too much furniture, over damped room ); voltage of source, feeding the amplifier; how isolated is the room, from extraneous sounds and noises. Tubes vs solid state, as speaker amplification.....I will stay out of it, because I favor good quality ss....
onhwy61  said: "If the loudspeakers are 87db sensitive -  Monitor Audio loudspeaker is a 4 ohm load." 

That is not correct the speakers are 90db and a 8 ohm load, 40 tube watts per channel should be plenty if not in a large room or at head banger volume.
Lot's of good info in the responses.  As a tube amp designer my 2 cents worth is that no mention of power supplies yet--and that is also a very key factor.  An excellent choke driven power supply can deliver instantaneous power which can be much more important than total RMS power.  Every amp is different and a 10W tube amp might outperform a 100SS amp, or the reverse depending on the total design.  

I once built a custom guitar amp for lead jazz guitarist which could only deliver 4-5 watts of continuous RMS power.  However, due to the design, it outperformed a 25 watt Fender amp and became his favorite gig amp.

try it and let your ears be the judge  
What a great thread this is! Lots of well-written, clear information.

My suggestion to the OP is to read Roger Sanders excellent white paper titled something like "tubes vs transistors". IMHO the information he shares will also help illuminate the topic under discussion.
Is there some law of physics I missed in school? This may be sorta true in some cases, with some amps but it's far from a universal conversion factor. I can think of many circumstances with my own gear collection where this is absolutely not true.
The missing bit is distortion. The reason the tube amp can seem to play louder than its watts suggest is that they have much smoother clipping character- rounding the output waveform when clipping rather than chopping it off as a solid state amp will do. So you may not hear the clipping at first but with a solid state amp you will.

As the tube amp approaches clipping it will generate more higher ordered harmonics. Since the ear uses these harmonics to sense sound pressure, it will sound louder at that point but a sound pressure meter will reveal what is really going on.
I asked Monitor Audio a similar question when I owned Silver 8s, but I was considering a 20watt/Ch amp. They said it would be fine. One reason MA recommends a lot of power is because their floortstanders are capable of very high SPLs. If you can high-pass them with a sub, that'll be much easier on the amp. 
Several have made accurate observations.  However, too much talk about how loud.  What about how the Mahi Mahi sound?!  I've run them for years and tube rolled them to their pinnacle performance.  They've run Merlins, SA, Epos 11, Reference 3A and more.

The EL-84 is really fast and provides solid controlled bass.  These are about quality not quantity.  Acoustic bass is very well defined, vocals are open and instrument attack is immediate.

And besides, can we all not listen to well done production at a lower level?  Oft times volume is used to compensate for poorly done reproduction.  These allow for very satisfactory listening at sane levels and will not produce insane levels unless you pair with horns.
celtic66, every recording has a "right" volume. And that right volume is multi-factorial. It depends on the type of music, the way it was recorded and mastered, the way your own system is equalized and the way our ears change frequency response with volume. Bass and treble increase with volume (see Fletcher-Munson). A recording that was meant to be listened to at loud volumes will sound dull and lifeless at low volumes. Turn it up and it sounds great but if you tried to play a string quartet that loud the violins would slay your ear drums. Rickie Lee Jones albums were recorded on the bright side and sound best (more natural) at moderate levels. Turn it up and her voice does that violin thing. I have an old Buddy Miles album with some great songs but it always sounded lifeless to me. I hadn't played it probably in decades. At 95-100 dB the record is a knock out. They were probably stoned and had the monitors turned up to the max when they mastered it. 
A word about listening to loud music. Always warm your ears up. In your middle ear you have a muscle called the Stapedius which dampens the Stapes bone keeping it from vibrating as hard against the oval window. Google it and you can see the anatomy. When exposed to loud sounds your brain tightens the Stapedius through the 7th cranial nerve lessening the amplitude the inner ear sees (hears) but it takes a little while to tighten. Don't just go right to 100 dB. Start off at 85 dB and crank it up 5 dB every few minutes till you get where you want. Beyond 100 dB is no man's land. It also takes a while for the Stapedius to let go which is why after a loud concert you can't hear anything for a while.
celtic66 quantity and quality are essentially the same or so closely tied that you can not separate them. The quality is not there unless the recording is played at the right volume which is determined by all those factors above. 
I really appreciate all of the discussion. My local store let me borrow the Mahi Mahi over the weekend and I spend a good amount of time playing with it and swapping back and forth to the Parasound A21. For additional context, my Preamp + Phono is also a Manley (Steelhead).

The room I am in is pretty small and the speaks only 8ft or so from listening position. I did not have any problems getting good performance out the power of the Mahis.... even in Triode mode things sounded great. UL bumped up the power but I suspect for most of my listening (jazz and acoustic vinyl) I’ll be playing in Triode.

At the end of the trial, I’ve decided to keep the Mahi Mahis and will end up selling him A21s at some point. Not sure if the Manley Steelhead being of the same manufacturer is part of it but the combo is sweet.

Now it's my time to crow; everything is just like I said, "Tube watts are louder than SS watts, plus they sound better"

Also, the room is the dominant factor; unless you got a huge room, you don't need all them watts.

When I'm right, I let everybody know it; but most importantly, we have another contented audiophile.
I rest my case.  Mahi Mahi mono blocks are so good.  And Manley Labs is correct.....Tubes Rule!  Never had a doubt.  Now you can play the tube rolling game with Upscale Audio.  

It is mind blowing how different these amps sound with tube and power cord upgrades.  I run Patrick Cullen.  You may like others.  The timbre, resolution and attack of the amps will change appropriately with upgrades.

The Steelhead is great and voiced well for the Mahi, but you may not need to go beyond the Jumbo Shrimp (no phono).  That was my match up until the Manley Neo Classic 300B with Takatsuki 300B tubes.  Enjoy the ride.  

You have done well grasshopper.
Orpheus10 , you are right. Tubes gives a louder sound, but more correct. When the tube-amp is 40 W, than he gives constantly 40 W. I had a Copland 405 tubes (25w) ,with the Audio Monitor PL 300 (350w): In a room of 55m2. It sounded incredibly beautiful. You gonna enjoy it!!!!  When you play exactly 10 W in your room ,after an hour ,  you go walk in the park ,because it’s very loud
Please correct me if I'm wrong as I am not an expert but I have always felt that that the higher current amps give much better control over the speaker. Damping also plays a critical role. I feel like watt ratings although important are usually a marketing term to ill informed consumer.
Please correct me if I'm wrong as I am not an expert but I have always felt that that the higher current amps give much better control over the speaker. Damping also plays a critical role. I feel like watt ratings although important are usually a marketing term to ill informed consumer.
Actually no speaker needs more than 20:1 for a damping factor and many need less. If the damping factor is too high the speaker is overdamped and an unnatural phenomena known as 'tight bass' occurs. Many audiophiles like tight bass, but its a coloration.
I can tell you on my experience using 50 watts manley stingray with thiel cs 6.3 and even with larger cs 6s. Stingray perfectly drived cs 3.6 s for many years. Later i used 200 and 300 watts solid state amps with the same speakers. I cannot say that there was a luck of power from stingray
Actually no speaker needs more than 20:1 for a damping factor and many need less. If the damping factor is too high the speaker is overdamped
True, above 20 it practically doesn't matter.  What difference could possibly 8/20 = 0.4 ohm make, when you already have about 6 ohm in the circuit (approx. 6 ohm is resistive impedance of 8 ohm speaker). 
Difference between DF=200 and DF=20 is like 6.04 ohm vs 6.4 ohm (6% difference for damping).   Difference between DF=200 and DF=2000 is even smaller  0.6%

The bottom line is “ You won’t know until you try it “. However , I went down this Rabbit Hole with a 30wpc integrated and speakers that needed 40 watts , but were more at home with 200. In my scenario it sounded fairly nice , but couldn’t play loud or have any Slam . You need a bigger amp/amps . Something like Rogue Mono Blocs that have a SS power supply and KT -120’s. Go big or be disappointed . The Manleys are somewhat particular , so read up . They would be great with some DeVore’s or some other dynamic , easy to drive speaker . A pair of Bob Latinos 120 watt monos would be sweet with your speakers and very affordable. Cheers , Mike B.