Jsman,
That is a tough question. It is not so much a case of some huge difference, as so much a matter of degree. In many respects differences between tube types-- 45 vs. 2a3 vs. 300b vs. 211 -- is more significant, and arguably difference between brands can be on the same magnitude (for example an EML meshplate 2a3 will sound VERY different from a JJ meshplate 2a3).
If one is comparing "like" amps-- pushpull 45 vs. SET 45, or PP 2a3 vs. SET 2a3, the SET amps tend to develop a more enveloping soundstage so one feels more emersed in the sound. A really good SET amp is surprisingly dynamic and lively sounding at lower volume levels (the lower one goes in volume the bigger the advantage of SET amps). Good SET amps sound relaxed and lacking in artificial edginess, while to sounding murky or sluggish.
But, good pushpull triode amps, are pretty good too in all respects. The sound of such amps tend to be punchier and tighter (less overhang on notes), and hence, they tend to have a more driving rhythm. This is good if one's system is a bit too polite and laid back. But, an over emphasis on rhythm and drive can give the music the "mechanical" quality I mentioned in an early post. So good quality can become bad, depending on ones taste, system, perhaps type of music, etc. This tighter sound also means that notes seem to decay faster and so there is less of a lingering "wash" of sound (again, good or bad is a matter of taste).
I happen to like both pushpull and SET triode amps. I certainly tend to prefer such amps over higher powered pentode and tetrode pushpull amps. Most of these kinds of amps sound a bit too brittle and edgy to me, and they tend to be lean sounding (lacking in full saturation of all the harmonics). Also, they tend to sound dead and lifeless at lower volume levels.
I am pretty much a low power amp fan. To me, one can tell how good an amp is by how LOW one can go in volume and still enjoy the sound.
