i recognize that there are a FEW good tube amps out there, that MAYBE would meet or exceed a standard for low distortion (if there was such a thing), however IN GENERAL they create that "sweet/good" sound due to the characteristics of their distortion. that is why they make different brands/types/etc. tubes to distort the sound differently. this point is painfully clear to everyone, so there needs to be no more discussion about it.
as far as bi-amping or bi-wiring goes. there is absolutely no need to bi-amp passive speakers, and if given the choice one should bi-wire.