I've been faffing about with different types of valve amp for a few years, no sort of expert but a reasonably experienced faffer.
You'll need very good ears indeed to hear much change between various types of output valve. Once you get above the lowest-quality units, a valve is a valve, IMHO.
Input valves are different. Like wines, they attract all sorts of fanciful adjectives (smooth, creamy, crunchy, sparkling) which actually tell you very little until you've heard that particular valve used in your amp, with your bass played through it, by you.
There's a shedload of information about pre-amp valves on the Net but be warned, most of it is aimed fair & square at guitarists rather than at bass players.
The ECC83 is the most widely-used pre-amp valve, and it's also commonly known as the AX7.
If that's too high-gain (i.e. breaks up easily into distortion) for your tastes, try the next step down which is the AT7. That's a roughly 40% drop in gain level leading to a much later break-up.
Watford Valves have attracted mixed reviews on Basschat. I really like them and have had nothing but good service from them, but I am aware that others have had different experiences.