| well it could be a number of things.
do you feel you have a good sound live? or are you experiencing some of the same problems live that you are in the studio? if you are, then the problem lies within your own sound.
if you're not.....
change your strings if you feel the sound is too dead.
make sure you have a good engineer who knows what they're doing when it comes to tracking bass. What style of music is it? there are so many combinations of compressors, pre amps, cabinets, amps, mics........
you need to get the right combination for the sound you're going for.
when you say you've been into two different studios, do you mean you've only recorded your bass twice? if so, there's a long way to go with finding the right sound. It takes a lot of work being in the studio and searching for sounds over time.
As a basic rule of thumb (and I'm just geussing what the studio might have for outboard gear...) you can get a pretty warm sound out of an Avalon 737. make sure you start out with your bass flat on the eq and try and get as much of a natural sound out of the instrument. start from flat and work with the sound until you get it in the ballpark. try not to mess with the EQ too much on the avalon or which ever pre amp you might be using. and also be concious of the fact that the sound of the bas on a recording has a lot to do with where the mix engineer places you in the mix. you may well have thought the sound of the bass sucked because the mix engineer didn't have a good relationship in the mix between the kick drum and the bass. there are just so many things that go into making a good sound in the studio.
the more basic you can be with your raw signal to the tape or hard disk the better. If your raw signal is clean and you can hear everything going onto tape then you know what you have to do with it yourself. Hopefully you won't have to make any changes to your natural sound, but you might find there are some inconsistent things in your playing that might not be totally condusive with recording.
Recording is such a different beast. And everything goes under the microscope way more than a gig.
I would also recommend that you (or as a band) employ a good producer who has a good working knowledge of the studio you're working in.
I wouldn't blame it on the bass you're playing..... I've played a mexican P bass which cost $289 on albums and it's sounded great. I just played it on my new record actually, so it's not really about the bass, it's about the sound of your fingers.
Easy,
Janek | 
