Very cool! Thanks for the quick and direct info, Mr Deckert!
So I tried full bore on the ZBIT, had been running the amp high and it low. Wow! I think I never even tried it this way as my plan was all along to use the ZBIT as a passive preamp for volume control with some range up and down.
I will continue experimenting but I think all 5 (now wishing for more!) volts from my source make a big difference when the volume is managed at the amp even though the amp may never run over half way up. As though there is just more information getting through. Ima have to train my sausage-fingers to operate the two volume controls in tandem!
Maybe I can get a ZBIT made to order with no volume control at all... or with just a mute/dim switch? I am so lazy, turning the ZBIT knob all the way around and down quickly for the rare phone call (so as not to mess up the careful balance set on the amp) will surely be a chore

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And Lon and LR, thanks for the reminder. The 25th is surely the best sounding of any audio device I have owned. Although 'best' is too small a word to describe its immersive, mind-bending impact. Even through my modest speakers (Heresy III's) I am hard pressed to recall ever hearing more 'better' (even when having snuck into the appointment-only room ages ago when there were hifi stores local to me). Maybe the DNA2's, or the ZOB's... I really like the single driver idea.