PX-25 tube amp with Merlin VSM speakers?

Brf nailed it. Quite simply, the Merlin is not suited to most SETs due to their efficiency. Dazzlingmd, your room is only a little smaller than mine, and my speakers are almost 10 db(!) more efficient, yet I find that its nice to have the extra power if I really want to enjoy the speakers. Take that extra 10db away (equivalent to 10x the power) and its just not going to work that well.

If the speaker requires that you are push an SET past about 20% of its full power on peaks, you will not be hearing all the amp has to offer and likely not all the speaker has to offer either. In this case, because the speaker is a medium efficiency, you really need more than just 7-12 watts to really make it play. The bigger you make an SET, the more the bandwidth is degraded by the output transformer.

The advantage of an SET is that it has distortion that linearly decreases to unmeasurable as power is decreased. When you remove distortion, detail is revealed. This is why SETs are known for their great 'inner detail'. However the flip side of the coin is if you want to get everything out of the amp that it really can do, you should not exceed 20% of full power, since above that level the higher ordered harmonics (to which the ear is very sensitive) come into play. That is why the best SET installations often include horn loudspeakers.

You may encounter people that may try to convince you otherwise, but IME (The Merlin is the most popular speaker used with our M-60 amplifiers) you really do need some power to make the speaker sing, even if only a moderate amount. The speaker is very revealing and has surprising bandwidth; what I am saying here is that the performance of the amp won't be realized neither will that of the speaker if you use a low powered SET on it, unless you are in a very small room.

Dazzlingmd, the difference in a P-P triode amp making 15 watts and an SET making 15 watts is that if you want to hear what the SET does, its likely you should not push is past 3 or 4 watts. Otherwise the higher orders come into play.

(The higher-ordered harmonics behave as loudness cues to the human ear/brain system, causing the SET to sound more 'dynamic' than it should be for such low power. This is something you will often see in reading reviews of SETs- its a psycho-acoustic interaction; once you realize that is has to do with how the amplifier makes distortion, its also easier to hear the amp doing it, so in a way I may have ruined it for you by writing this...).

With a push-pull amp this is less likely. So you get a greater percentage of usable power.

With our amps, as they have little or no feedback but are a form of push-pull, they too have a greater percentage of usable power, but are also quite relaxed in the mids and highs, in fact more so than the typical SET because they are lower distortion. Yet they also have more detail, owing to the lack of an output transformer. The combination of our M-60 on the Merlin is quite good, which is why the Merlin is one of the most popular speakers used with the M-60s. What is nice is to have 'highly detailed' in the same place at the same time with 'relaxed presentation'.