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.
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.