Bob, I was wondering the same thing - the common opinion over at DIYAudio and Audiokarma is "Before going to the trouble of replacing your VTA / Poseidon / whatever board with the original Hafler board, you want to go listen to an original ST-70 - sure it sounds good, but in a 'Fuzzy blanket' sort of way - it sounds DEAD".
Sorry, but I choose Accuracy and Detail over colouration and fuzziness - even if it means that it exposes weak components in the rest of the system.
Regarding NFB: I always wanted to ask about Negative Feedback (but were too scared in case it leads to a Religious War)! Bob, seems to me there was a line of thought in the Audiophile world in the 1990s/early 2000s that NFB is a BAD THING and should have been banned by the Pope, the UN Council on Human Rights as well as the SPCA. I therefore find it VERY interesting to know that the VTA board will still stay (relatively) stable when the NFB line (the 16 Ohm output on the OPT IIRC) is disconnected.
Supposedly, running without NFB should have the same effect as running in Triode mode (versus Pentode/Ultralinear mode), and I have been meaning to try it. I don't think I will bother now....