Hey Dave,
I obviously can't know, but it is possible that in at least some of those instances when you heard a haze it was either the transformers, as in PrimaLuna, or carbon resistors in others. I think a lot of people like that haze, it's why, IMO, Vandersteen speakers sell so well. That haze helps cover other issues, for some it makes music sound good, for others it obscures detail.
The PrimaLuna I had was WAY under biased, so the KT88's wouldn't have been operating in their most linear range, but even with my current amp they are a little under biased, the power transformer can't put out enough voltage for them, and the output transformers are set up for EL34's, so they too are less than ideal. I may well put my KT88's back in it until I can upgrade the transformers as I think KT88's are some of the best sounding output tubes there are. I hope to address that as time goes on.
I know that PrimaLuna brags about their transformers, but I am less convinced, people who are much deeper into tubes than I apparently think they are not very good, and merely looking at the physical size of them makes me think that they were not made for KT88's at full power, which ought to be about 70-80% of the 42 watts each that they are rated for, so about 31-32 watts per tube will give sufficient bias, and not burn up the tubes prematurely, in other words, they will be running in class A/B rather than class A bias.
Anyway, that MAY explain at least SOME of this issues that you have run into with tubes sounding off. The most expensive part of any amp, that doesn't use a switching power supply, is the transformer, so if you want to skimp on a tube amp, skimp on the transformers, actually the same can be said of transistor amps, and may explain the lack of bass in some of them. Anyway, we may well never fully resolve this question, though the newer Kiwame and Takman Rex resistors supposedly use carbon film, and have less noise than many metal film resistors, so people may be able to "warm" up their amps without sacrificing detail now.
