It's hard to argue against any Audion amp to be used with Zu. The Silver Night 300B monoblock power amps are available either as PSET or P-P. 18/18w is fine with Def4. The issue is the bottom end. Look, I have the Golden Dream PSET 300B Audion top-line monoblocks. They are a big step up from Silver Night, with more controlled bass and still, the 845 Black Shadow surpass the Golden Dream on deep bass performance -- and that's after a recap mod to further improve Golden Dream bottom end discipline over stock.
Take your pick based on what's important to you: The 300B PSET amps will have greater ultimate resolution and tone density. The 845 SET amps will have greater drive, more defined and punchy deep bass, and mids/highs will be still meaty, toneful and strong. If you were asking about Superfly, limted to a bottom response of about 30 Hz, I'd say go for the 300B over 845 option. But for Defs, the 845 bottom will underpin a more complete dynamic realism and give you nearly all the midrange magic of 300B SET, with more shove in tonal events and indisputably better dynamic life. If you like maximum presentation of delicate information and don't mind some ripeness in your bass, then by all means choose the 300B PSET. A compromise would be to get more like the 845 bottom end in the 300B push-pull version of Silver Night, but that's at some expense to the holism of SET.
I sometimes use my outstanding Golden Dream pair on my Defs, but by and large, they do tone-drenched duty on my Druids and the 845s put in ceaseless duty pouring watts into Definitions. But if you can, listen to Silver Night 300B vs. Black Shadow, and tell me what you embrace?
I've heard unmistakable advantages to TVCs in place of active preamps on a variety of power amplifiers. On paper, the advantages should be uniform, but in practice they are not. All I can tell you is that I've tried a few high-grade TVCs into Audion power amps. Not even one of those combinations tied with or bested the sound I got from the same Audion amps fed by my extant tube preamps. The combination of TVC + Audion power amp certainly did not sound in any way poor, but compared to other TVC + amp combinations where the resulting sound was a clear improvement, the Audion + TVC combinations failed to satisfy, even sometimes setting sound quality backwards a bit compared to pairing with a fine active tube pre. It's possible that if you rewire the inputs on the Audion power amps to bypass the input potentiometers that the TVC might become preferrable, but then the volume control range and precision may not be adequate, given the high input sensitivity.
You should try it yourself and see which combination you prefer -- no harm will come from disagreeing with me. But if you are asking for my recommendation, my answer is to find best satisfaction, on balance, via Black Shadow over Silver Night, and active tube pre over TVC with Audion power amps, *on Zu Definitions.* If you decide the 300B amp better matches the attributes you value most but you find bass performance the only area of doubt, you can either have a technician perform a recap of the power supplies to improve bass, sacrifice some single-ended tonality in favor of push-pull control, or live with some bass bloat and do your best to tune it out of the Def4 sub via the subs' performance tuning controls. Regardless, these are differences of degree. Any of the Audion amps will be exceptional, and whether to TVC or not to TVC is up to you.
Phil
