Aha! You were withholding important information! That changes everything. I take back everything I said above.
You really, really, want a B100R. It's a little heavier than you probably were hoping for, but you can definitely throw it in the trunk or back seat. I suggest it for two reasons:
First, tone. If you like that Ampeg sound you get from your big rig, you won't find anything near it in any other small combo. The B100R is designed to emulate a B-15, not an SVT, but you will love it. You'll be amazed at how rich and tube-y the sound is for a solid-state amp. I play my Jack Casady sig -- a kind of hybrid between hollow and semi-hollow -- through mine, and I swear it sounds like the amp and bass were designed for each other.
Second, size/power. If you get a little 50-watt something, it'll work for some of these "low-key practices" -- say, with an acoustic guitarist -- but there will be a gap in your gear between that and your big rig. For a practice or jam with a drummer and one electric guitarist, for example, the big rig will be more than you need but the little one will be inadequate. (I had a B50R before I got the 100, and although the tone was great it didn't have enough balls for those sessions. And, it couldn't handle the B string on a fiver.) If you get the B100R, you can use it for everything up to the point that you need to go big, and all your bases will be covered.