You could also look at the Crown XLS drivecore series. They can be ran with a VT pedal too, if on the cheap. You could also look for used Peavey preamps. I got an Alpha Bass Tube preamp for $100 used, locally. I also got my Crown through Northern Lights and Sound. They run a 15% off coupon every so often and thats the time to strike. Of course, finding one used would also save a buck or two. That should leave plenty of room to have a few bucks left over from your $600 and you still have cabs you could put down as a trade. Or sell the combo and get another 4-10". There are really a lot of possibilities.
Knowing where you are will help us determine gear availibility. Assuming you are in the U.S., there are a ton of local outlets for used gear including TB.