You don't need a valve amp IMO.
I'd go and try a few, but ANY new bass amp in your budget is more than capable of making a pleasing sound, the challenge will be finding something that really pushes your buttons.
Personally-speaking, I'd look for a head and cab - more versatile and also more scope for experimentation. I'd try all the usual suspects but for £800 you should easily get something that'll do you proud.
Cheers,
James