Hi DavyGravy3, if you're looking for a tube amp for bass, I'd suggest something with 6550 output tubes. Those tubes slam, they're punchy and dynamic and they give up plenty of power. 6L6's would be the second choice. EL-34's would probably be the last choice for bass, they're good for guitar but (generally speaking) they break up too early for bass, and they're not nearly as punchy as the bigger tubes.
That being said, the second question is how much power you'll need. It's true that 300 honest tube watts generally sound louder than 300 solid state watts (the reason is somewhat intricate, but in a nutshell it has to do with the way the amp responds when it's driven hard, in a tube amp the extra drive goes into "tube overdrive", meaning the average power tends to stay higher over time, while in a solid state amp the extra drive goes into "clean headroom", meaning the peak power is higher but the average power tends to stay down low).
If it were me, I'd be looking for something in the 200-300 watt range. That would be good for most indoor club situations. If you're doing outdoor gigs, you might need a little more power, in that case 400 honest tube watts wouldn't be unreasonable. Of course, the downside here is that any big tube amp like that is going to be HEAVY. Figure 80 pounds at least. Definitely watch your back.
You can probably get a used vintage SVT in the thousand dollar range. At that price, it won't be in "mint" condition, but if you're patient and you find the right one, it'll be functional (if maybe a little ragged looking). Another thought would be a Marshall Major, I've seen those on eBay in the thousand dollar range. Either one of these will most likely use the 6550 tubes (or some variant thereof, like KT88's, which are kind of the "hi-fi" equivalent of 6550's). Either of those will give you the nice slammin' tube bass sound.
In the 500-700 dollar range, you might look for a component rig, like for instance a rack mount tube power amp and a separate tube preamp. If you're good with electronics and handy with a soldering iron, you can build a tube preamp for less than a hundred bucks, and you can make it sound just as good as an Alembic F1-X or a Kern or something like that. Building a good high power tube amp would be a little more difficult (and time consuming), but a preamp would be a piece of cake (relatively speaking).
Another thought would be something in the military surplus arena. If you search eBay under "vintage tube amp", you can find 200-300 watt military surplus amps that are well within your price range, or maybe even less than 500 bucks. You'll have to be patient, and jump on it when you see one (use "Buy It Now" if you can).
Good luck, and let us know what you decide on!
