Sounds like you've got the fixins for a right nice amplifier there! I've read several positive impressions of the Tubelab kits, and those Transcendars you're using are certainly nothing to sneeze at. That's important considering that (IMHO) 50% of a SET amp's sound is in the output transformer quality. Transcendars don't produce as much bass as some can, but once they're burned in well it's tighter, cleaner bass than some brands that cost much
more. They have great microdetail, and their midrange tone is simply superb by anyone's standards, one of the best IMO.
lousymusician wrote:Compared to my modified ST70 (which uses the same output tubes in push-pull triode), the bass is a little softer and 'rounder'......
You can expect that, as the ST-70 is using 20dB of NFB (unless you've changed it) and trioded power tubes, especially if you've fixed the design flaws in the feedback loop and put in bigger coupling caps. Its output Z is probably only a couple tenths of an ohm, versus around 2 ohms for the zero feedback EL34 SET (depending on the EL34s' operating point and the output transformer impedance ratio). Not considering output transformer secondary winding resistance or other variables, that's a DF of 40 versus a DF of 4.
Having said all that, the Tubelab's bass will undoubtedly tighten up a lot and gain significant extension with time. If the ST-70's feedback loop and coupling cap values haven't been modified from stock, I'd be willing to bet the SE amp will actually have cleaner bass once fully burned in, despite the very low DF (depending on what speakers you're using, of course).
Congratulations on a successful build!