While I have heard several Volti speakers (you did not specify your model), I have only heard one First Watt amp, the F2, I believe, in my own system. I had a friend's amp for about two weeks. I really liked the sound of the amp, certainly one of the better solid state amps I've heard. I also heard a friend's DIY amp which is based on a First Watt amp; I don't recall which model it is based on. Both amps are lively, natural sounding amps that don't have that dead, lifeless, quality that I hear with a lot of solid state gear when it is not cranked up to high volume levels. As compared to my SET amp (Audio Note Kageki) and my pushpull amp running 349 output tubes, the F2 had just a touch more of an artificial "edge" to the initial attack of a note and it delivered a slightly smaller and less enveloping soundstage. Still, it is a great sounding amp to me.
It is almost impossible to make any specific recommendations on SET or any other kind of amp. Each specific model, tube type, specific brand of tube, etc., can dramatically affect the sound. If you can, you should try to hear different models and tube types to develop your own sense of what you are looking for. While 300b is a favorite tube type of many SET fans, there are others who favor types like the 45 and 2a3. If more power is needed, the 211 and 845 and some even higher output transmitter tubes could be employed, but again, there are sonic tradeoffs with all choices you can make.
I don't want to confuse things, but, don't overlook pushpull amps and output transformerless designs. There are plenty of outstanding tube amps, particularly if you don't need much power, that are not SET amps.