Interesting take on the Zero....the reason why I went with the (K42 series) PIOs was to mitigate the hyper real representation that the WIMA MKP bypass caps gave to the unit...the PIOS if carefully installed with the leads as short as possible pick up no noise whatsoever and impart a lovely tube like vocing to the overall mod (addressing your main complaint for the most part)....the PIOS are critical to the end result (at least 50% of the voicing is at stake here). Combine that with the HDAM OPA and it's PIO mod and you have a very accurate and cheap source that can be modified quite easily by a beginner ( no small consideration when to take into account the complexity of adding a tube output stage and it's related PSU components).However I do like the idea of the tube back end and I would like to try that someday. I have a number of 6N6P-I's on hand gathering dust that could be pressed into service if I should decide to duplicate the mod.
Your NFB 11 mod is pretty cool.....Lampy is a unique individual and a great resource as long as you like tubes (for everything) and are well versed in DIY precautions when dealing with high voltages. The 6N6P is a good tube but it's hardly SOTA....for these purposes however and the target price points of these DIY projects I can think of no better candidate (tube) for those on a budget.
Good work T...
Thank you, Peete, for being inspiration at first place. I guess I didn't have green PIOs handy and could not simply wait few weeks for them to arrive from Eastern Europe - too exhausting, I'm sure you know what I mean . All the best to you!
P.S. Don't try 6N6P-I, those are not for audio at all. You need plain 6N6P or, better, get yourself 6N16B and try in SRPP or simply common cathode mode. Last one's improve like your PIO caps .