If you're playing bass with someone else then i personally wouldn't go below a 200W setup. Because of the frequencies you're putting out you'll need it to cut through a 50w guitar amp. Also, guitarists generally have zero idea of the importance of volume control...
If you don't want to have to mess about cutting up your old combo id suggest something like the TC Electronics BG250-112 which can be had for around $600. More than you wanted to spend (and you can't run a second cabinet) but will do what you want and sound pretty good.
Enjoy