I have both One and Two Pi systems. I've tried a 2 watt Yamamoto A-08 SET (45 output tubes) and it strains. I've had luck using a 30 wpc Cayin TA-30 (PP EL34) and it does a good job. Currently, I've settled on an 8 watt Almarro A205 (SEP EL84) and I think that's where it's at. Just to be stupid I tried the Two Pis with a set of old Baldwin organ amps, which are PPP 6L6s pushing about 120 wpc. Very nice - really showed what the Pis could do, but this was complete overkill for the bedroom.