Good choice Saffy,
The PX-25 is one of the finest amps I have ever heard. It has a musical quality that is entirely unique, one reviewer coined the phrase "as if instruments were lit from within". I agree.
The PX is 6wpc, but has current capabilities beyond the normal 6-10wpc SET amp. I suggest speakers of >95dB to really let the amp shine.
I'm partial to single-driver, or more accurately, wide-range driver speakers. They usually feature a 1st order, or no, crossover at all, which along with the single point of origin for the majority of the frequency range leads to a greater time and phase coherence than multi-driver speakers with higher order crossovers.
By "musical coherence" I mean that is as if all sound flows from a single musical tapestry. Not one dimensional, but a beautifully woven soundscape of depth and nuance. Done properly, it is awe inspiring and there is nothing like it.
But, that isn't the only, or even best, approach. Like everything in audio, which is best for you is a personal choice and may not be the same for anyone else.
Welborne Labs, which also makes fantastic SET amps, has a great page listing
speakers that mate well with SET amps. They also have a short primer on proper speaker matching.
The best advice I could offer for anyone interested in high-eff speakers would be to check here, Audio Asylum's
High Efficiency Speaker forum, and on the
Fullrange Driver Forum for a horn or high-eff listeners group in your city. At the very least, there might a few owners in your area. Ask if you can check out their systems. The more you hear the better.
Take your time before jumping in, high-eff systems are much harder to get right than most people would think. And enjoy the learning process.