Speakers for triode tube amps

I have a customer who is running Tekton's.  They can be driven and sound good with a 20w SET.  His space was a bit too big though.  They are not as efficient as you might think so I would not plan on sitting more than 8-10 feet from them and hopefully the room is not too big.  

Zu is a but more efficient, are a bit smaller than Tekton and sound very, very good.  I have had some in on trade and ran them with an 845 based SET and they were INCREDIBLE.  Same customer also has a pair of Zus and they are a bit more efficient and just incredible.  They would be at the top of my list. 

How big is your room?  My speakers have a min impedance of 7.3 ohms are are extremely tube friendly but they are pretty inefficient.  I run them with 45w at shows which is more than enough in a 13 x 20 room but...

None of these are horn loaded. 

I would tell you that Avantgarde Zeros XDs are incredible but they are horns.  They don't sound shouty like horns but it is a big, wide speaker which I am sure rules them out though they sound nothing like Klipsch or JBL. 

Russ69,, said - - " Unless you play loud a 25 watt tube amp will get the job done with just about any loudspeaker. "What an absolute load of RUBBISH! You are so SO far off the mark here that I'm amazed that you haven't been flamed over that dumb comment! Ridiculous!

There is enough wiggle room in Russ69's comment that it is not unreasonable.  First, "play loud" is not defined, and it IS the primary factor in whether or not a modestly powered amp will work with a given speaker.  The other bit of hedging is "just about any" which means there are indeed some speakers excluded.

I listen at modest levels.  All three of the amps I own are adequate, in terms of power output, and the highest rated output is the parallel single-ended amp rated at a whopping 6.5 watts/channel.  I have heard this amp used on much less efficient speakers than I own (my speakers are around 99 db/w efficient), so I know it has plenty of power for my purposes. 

Even at less than quiet listening levels, and even with low efficiency speakers,  peaks that would challenge a 25 watt amp represent only a small fraction of time.  Whether or not the compression or distortion at these peaks are so unacceptable that the amp is disqualified is a personal matter.  I would live with occasional problems at extreme peaks if I otherwise liked the sound, others might not. 

A 25 watt tube amp, particularly one with a higher output impedance, might not be the best choice for listening at even modest levels with some speakers, so there might be an increased probability of incompatibility.  But, matching an amp to a speaker involves so much more than having adequate output, decent damping factor (low enough output impedance), etc.  There is no way to really know until the combination is actually tried.  The Original Poster mentions having Harbeth 40.1s, which, from hearing it with around six or so different amps, I think it is a challenging speaker.  But, interestingly, it happened to sound best, to me, with two amps that had lower output than 25 watts, while the 100 and 200 watt amps did not sound as good.  Yes, both of the lower powered amps compressed a little at high volume peaks, but, for the vast majority of the time, they sounded terrific.  

The question is what "will get the job done" even means.  Yes, it will make sound but it was so spectacularly uninformed and shows a real lack of experience witnessing amp/speaker synergies that I just bit my tongue at the time he posted that comment.