Most bassists will agree that an all-tube amp will be louder than a solid state amp of equall wattage. Note that all-tube means tube pre- and power sections, not just the preamp. Most typically, all-tube means Ampeg SVT or Mesa 400+.
The main reason is that you can drive a tube power section into clipping and it doesn't sound awful. Depending on your tastes, it even sounds good. Tube distortion is just a little gritty, very "punchy" sounding. A solid state power amp driven to clipping sounds just awful, and that's not really just according to tastes, that is a pretty universal opinion.
What this does is it allows for a much higher average wattage from the tube power section; since it's OK for the peaks to clip, you can run a much higher average level. With a solid state power section, the entire signal must be reproduced without distortion, so the level must be set accordingly.
Also, tubes have a natural compression effect that enhances percieved loudness.
Tubes also tend to accentuate midrange frequencies a bit, which contributes even more to the perception of loudness.
So, all that sounds great....why aren't all bass amps all tube?
An all tube bass amp that puts out sufficient wattage for an average rock band is very heavy.
And due to the components necessary to build them, they're expensive.
And they're very heavy.
Did I mention how heavy they are?
Truthfully, if that's what you're looking for, the only way to get that true tube tone is with an all tube amp. The drawback soundwise is that tube amps are a bit less flexible than solid state amps; they pretty much will always have some of that tube flavor, whether you want it or not. Solid state amps faithfully reproduce whatever tone you feed them, and while you cannot get a "perfect" emulation of an all-tube amp from a solid state rig, you can get about 95% of the way there, while still having the options available to a clean, fast amplifier.