100W, 200W, or 300W?

I'm pretty new to this and could use some help. Working down my list of upgrades: did speakers, room treatment on order... I think next up is to replace my amp.

I'm currently using a Harman Kardon PA2000 stereo amp that I had sitting in storage. 100 WPC @ 8 Ohm. It's "OK", but it probably the weak point in my system right now.

For speakers, I picked up B&W 805D3 tabletops with stands.

88 dbSPL sensitivity, 8 ohm. They recommend 50-120W @ Ohm for the amp power on their site.

I pulled up an amplifier calculator and plugged in some numbers:

88 sBSPL, 8 ft distance, 85 dB volume w 15 dB headroom and this came out to 3W RMS w 94W peak

I have my heart set on blue watt meters so I see the following options:

1) MA5300 @ 100 WPC, no autoformers, integrated
2) MA7200 @ 200 WPC, has autoformers, integrated
3) MC302 @ 300 WPC, has autorormers, seperates

For the MA5300, I'm concerned that there isn't enough headroom. If I even get close to the 94W peak, it means that I'm pushing the amp to the max, so I'd probably be operating in an area of reduced sonic performance since it's being stressed. Is this a correct assumption?

The MA7200 looks like it'll leave plenty of headroom and it also has (for better or worse) autoformers which seems to be what puts the "mac sound" in macs.

The MC302 is just sexy as heck... but is there any realistic gain with my current set up that I would get by buying one of these? Or is it so much overkill that I am just throwing money away at this point?
Most of the time, amplifiers are only using a fraction of the output that they're rated for.  Take the case of amps richly biased into class A: often they never leave A to slide into AB.  Up to a point, some increased headroom is good--think V8 over a straight 6.  The 302 is overkill: horses for courses.  BTW, B&W and Mac are often sold in the same store; is Mac what the store is pushing you towards, because you can do better for your $$?
For speakers, I picked up B&W 805D3 tabletops with stands. 88 dbSPL sensitivity, 8 ohm. They recommend 50-120W @ Ohm for the amp power on their site.

... For the MA5300, I’m concerned that there isn’t enough headroom. If I even get close to the 94W peak, it means that I’m pushing the amp to the max, so I’d probably be operating in an area of reduced sonic performance since it’s being stressed.
More significantly, IMO, you would be pushing **the speakers** to near the max. Note that B&W’s recommendation for amplifier power, stated fully, is:

50W - 120W into 8Ω on unclipped programme

Meaning that its ability to handle **continuous** power (which is how the amplifiers are rated) is certainly much less than 120 watts, at least if the speaker is to provide reasonably undistorted sonics. Which also seems consistent with the fact that it is a small (28 pound) two-way speaker having a 6.5 inch woofer.

Finally, there are undoubtedly a great many design differences contributing to sonic differences between the three McIntosh amplifiers you are considering. I would not let your decision be dominated by the presence or absence of autoformers, or by any small handful of design characteristics. I would start by researching what amplifiers others use with the same or similar speakers, and what speakers others use with these particular amplifiers.

Good luck. Regards,
-- Al

P.S: Note also that the MA5300 has a specified dynamic headroom of 1.8 db. Which means that on the peaks of typical "unclipped programme" being played at high volume the amp should be able to provide approximately 150 watts, not just 100 watts.

In other words about 25% more power than the speaker is rated to be able to handle, even on a short-term basis. Which seems to me to be a reasonable margin.

-- Al

100 watts per channel could be enough but not that particular Mac. It sounds strained at higher volumes compared to other more robust 100 WPC amps.
200 WPC should be enough, I have heard that combination and it sounds excellent, the Mac takes the edge off the diamond tweeter nicely.
the 300 WPC amp would not be overkill but for your listening distance it will be more of a luxury / icing on an already very nice cake.
As a lesson more power is not completely about more volume. It is about sound quality and life-like dynamics and speed at lower volume too.  Think very good bass and attack at low levels.  The amp makes this happen.  
Some people like the jump factor dynamics in high efficiency speakers - you can get this with lower efficiency and higher power.
While the 7200 will be very nice, my ultimate recommendation would be a Mac tube preamp paired with their 200 WPC power amp. Boosts the spend but sounds like you have it, this would make your system golden.
Get the biggest best amps you can.
300W RMS, or larger.
 You need the headroom, said,many times, when crescendos, drum, or guitar solos, or the song ending slam at the end does happen, you will need the headroom.
I would agree with this statement.  I also have the B&W 805 D3 (previously had D2).  I tested two amps side-by-side with these speakers.  One set were 250 watt monoblocks and the second set was 500 watt monoblocks.  The were the same product line from the same exact manufacturer.  The 250 watt amps were nice, but if you listened closely, the smaller 250 watt did not have quite as much authority with bass (not quite as full bass and not quite as much slam).  It was almost splitting hairs, but the difference was there.  I think a 50-100 watt amp would sound even weaker (but it would still work). 

You don't buy big amps for the total power.  You buy them because they have huge power supply capacitance and transformers.  You will get a lot more punch, slam and midrange/midbass body.  The smaller amps with smaller transformers and capacitance just will not have the authority, EVEN AT LOWER VOLUMES.

The McIntosh sound is more of a mild and laid back signature.  If you say the autoformers create the McIntosh sound, it could be that the autoformers are responsible for the mild/laid-back sound.  Just a thought.  Not arguing against McIntosh.  It's entirely a personal preference.

Just food for thought.