Set amps ?

I agree with Charles, 6 to 8 watts won't do it. A 20 to 25 watt SET might be enough, as benign impedence, can be more important than sensitivity, with tubes. If you go that route, it would be very important to try the amp with the Proacs and in your room, before buying.

I have had a couple of 20watt SETS and can recommend the Ayon Spark, which I use at the moment. Almarro do a nice SET amp, at a lower price, both are excellent. The Viva Solista I used before, is very expensive now and I think the Ayon is as good.

6 watts will work fine a lower levels in a small room, but if you want to bring these speakers to life consider a hundred watts or more.

I don't believe I would never partner any Proacs with SETs. The brand has always loved power. I agree with Tmsorosk - 100w plus.

Regards,

What is so great about them is that they are usually a very simple design employing Class A operation which can pass a less bastardized audio signal to the speakers.

What's not so great about them is that they are low power (you asked about 2 - 6 watts) so you need to mate them with the right speakers. From the responses above I don't believe the Proacs are the right speakers.

A simple design rarely equates to better sound. The bastardization occurs when designers don't implement circuits correctly.