The SET amp question.

Well that's a pretty good size space but 102 dB is pretty darn efficient. Its not quite double the size of my room (14 x 23) but I do have 8.5' ceilings. Assumming you've got 7.5' its less than 50% larger, but lets consider it twice as large for now. I found 9wpc 300b amps fine for moderate volume singer songwriter, soft rock, acoustic jazz stuff but the amps ran out of steam with more heavily layered material at somewhat higher volumes; with 89 dB Merlins (flat 6 ohm impedance). IIRC, each 3 dB increase in sensitivity is equivalent to doubling the power. So 89-92-95-98-101 That's about 16x more effective power, so the 300bs should be just fine and maybe even the 2A3s. There are lots of 300b amps out there and the cost of decent power tubes has dropped.

Swampwalkers calculations are close, but it is actually 20x so 2a3s and 45s are even more reliably "in the ballpark." It really is a matter of taste. 2a3s tend to be more lean (less upper bass/lower midrange) than 300bs, but more open on top and detailed sounding than 300bs (to me 300bs have a kind of blub, blub, blub bass response). 45s are sort of in the middle between 2a3s and 300b, in terms of leanness, and have a MUCH tighter bass response. They sound punchier in the bass than either actually, aside from the much lower overall output.

The overall sound of any amp can be altered somewhat by the choice of output tubes and the choice of the other tubes, so there is a range of adjustability. Also, with some 45 amps, you could replace the 45s with 2a3s, and perhaps adjust the bias for a little more current; this would mean running a 2a3 conservatively which would mean longer tube life. A friend of mine takes a 2a3 amp and biases the current down a bit and runs 45s hot. He likes the sound of 45s this way, but the tubes don't last very long (he has MANY spare 45s). There are lots of possibilities.

I am using a Yamamoto A-08s with my Zu Def 2s. With the original NU 45s, the amplifier struggled with large scale orchestral and choral works. The Emission Lab solid plate 45s that I now use give it a lot more oomph.

SETs get their capabilities from 3 things: Class A, triode, and reduced distortion at low power levels in the output transformer.

Push Pull amplifiers actually *increase* distortion as the power level falls to very low levels on account of the somewhat different behavior of their output transformers. Anytime distortion is increased, the masking effect of the human ear means that detail is obscured.

IOW its all about the transformer (its no problem to build a push-pull triode class A amplifier). But- take away the output transformer (get rid of it) and the argument against P-P vanishes.

At only 102db, unless you are just fine listening at relatively low levels, a 45 will not really produce enough power to be lifelike, although 45s otherwise sound quite impressive. It happens that in general, the smaller you make an SET, the better they sound (wider bandwidth). 10 years ago the 300b was king; 5 years ago the 2A3 had taken over but the 45 was already on its ascendancy...