If you've already got a nice valve amp, then an Ibanez Bass Tubescreamer might work perfectly . I managed to get my hands on one a few months ago - a bugger to track down, I can tell you, but definitely worth the effort. I haven't had a chance to try it through a valve amp, but even through a SS it's perfect for soloing; if you play with the settings it can do some fairly serious dirt as well.
[media]http://www.youtube.com/watch?v=Lgaoodi33mg[/media]