I play bass through an Ampeg V4. I've had it since for 16 years now. I wouldn't dream of replacing it. HOWEVER, my general thought on upgrading gear, particularily once you hit the pro mark, is to get what you need, when you can afford it.

I always ask myself, do I need this? Is there enough scrill rolling in from gigs or whatever to justify it?

If you're playing in small clubs and realise your amp doesn't have the drive to fill a deep/long venue, you need something new. You probably also have the funds coming in to accomodate it.

If you're in a basement or jamming in a garage once a week, your Mesa will fit the bill. I agree with Dean, tube rolling or upgrading will breath new life into the amp.

Besides, you have a killer amp. Why the lust?