Yeah, I think you did the right thing by saving the $300.00. You can get new GT 6L6GEs online for about $80.00 a pair. Might as well start fresh, especially if the rest of the amp is in "REALLY rough shape". I use Sovtek 6L6WXTs in my Super Twin head (essentially the same amp) and they sound great. You can get them for $35.00 a pair; less if your amp tech buys them in bulk, like mine does.
I use my Super Twin with a Fender 215 Pro cab and, volume- and tone-wise, it's useable even in a living room jam seting. Of course, the sanity of moving 150 pounds of amp and cab for a living room jam is a whole `nother question...
If you're looking for SVT-style power tube distortion at low volume, you probably won't be happy with the Studio Bass. You can get some good preamp tube breakup by cranking the "Volume" knob or hitting the input with a hot signal. However, to my ears, that distortion sounds much different from an SVT running WFO. With a passive P-Bass, I get the best results using a good distortion or overdrive pedal in front of the amp and setting the volume dial at abour 5-6. I've been using an SFT and Barber Deep Fryer, but I want to try a VT Bass to see if I can get some of that missing SVT tone. You will, however, have clean headroom for weeks.
As far as the impedance goes, the Studio Bass can handle a variety of different loads, but someone with more expertise will have to chime in on that. However, if you hook up both of your 4 ohm cabinets in SERIES, it should give you 8 ohms.
First, though, take the amp to a good tech and have him go through it top to bottom. It'll be money well spent...
