Most modern bass amps are geared towards more modern tones, which generally depend on their cleanliness. Lets face it, fuzzed up slap funk wouldn't work as effectively as sparkly clean slap funk... (discuss...).
Rock (and Blues to a degree) tones however thrive on a bit of "push" from valves, which is why there are a lot of solid state power amps that have a valve pre-amp section (Marshall Valve-State/Ashdown/etc, etc). Ampeg's SVT's are pure solid state, with the exception of about 2 or 3 heads which are pure valve, and a couple that are hybrid's. Don't know of any Bass Valve Combo's though apart from hybrid's.
Bass Valve Amps: Ampeg SVT2 & SVT Classic, Trace Elliott V-Type V8, Marshall VBA400, Mesa Boogie 400+, Orange's new Thundaverb 200. Pick of the bunch for me would be an SVT2 or SVT Classic.
IMO FWIW: Valve Bass Amps rule, run them on full volume (of course!) for that awesome power valve distortion.