As a general rule, I have found that MOSFET amps tend to provide "one note" bass that sounds like a wet pillow being struck by a canoe paddle. The sort of bass heard on early Telarc recordings. Bi-polar designs seem to be much better in this respect. Over the years, I have found that the biggest challenge in hi-end audio is getting good, tight, deep, natural sounding bass. Attaining air, depth, detail, extended highs and natural timbre are all easier by comparison.
amplifier's "slam-factor"
I wonder if anyone can explain me why there are differences in the so called "slam-factor" between different amplifiers (with comparable power ratings). It is well known that for example Krell amplifiers have a high slam-factor, while Mark Levinson amplifiers are quite tame in comparison, even the most powerful ones (> 300 watts per channel). Spectral amplifiers are very fast regarding signal amplification and transmission, but I find them not very "powerful" sounding (high slam-factor), assuming speed is one of the factors which determine whether an amplifier has a high slam-factor or not.