Set amps ?

That Speaker is considered an easy load at 8 ohms with a dip to around 6.5 ohms in the lower midrange region. The sensitivity is only 83 db as measured by stereophile.SET amplifiers range in quality and performance as do all amplifier types.Given the very low sensitivity of 83db, the room size and your listening levels will matter a lot with the lower power range amp(limited volume and headroom) you`re considering.

The better SETs have robust power supplies and premium quality transformers(along with better parts,wiring etc.) as opposed to lower quality SET amps.
You `ll likely have much better sucess with a higher power SET,certainly more flexibility,The better built SETs can sound wonderful with the appropriate speaker match.
Regards,

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.