Yeah, I guess asking a 12AT7 to be an output tube for bass is asking a bit too much.
I actually have a 5W Gretsch G5222 combo (exactly the same as the Fender Champion 600 except it's covered in tweed, and therefore much cooler) that I use for filthy blues harp playing. It's great for that, but when I unplug the speaker and run it through my bass cabs, it's not so great. The transformer wants to see a four ohm load, and plugging in two cabs isn't the most practical thing for easy, low-volume practicing. Also, it sounds pretty dark with the 12AX7 and 6V6. Maybe a lower gain preamp tube would help, but I'm kind of just looking for something different.
Actually, I've been really interested in the BATT and also the SWR Interstellar overdrive for a while now. They both seem like they would be awesome, but I have never seen one of either come up for sale locally, so I could try it through my rig first. I'm decent with a soldering iron, so I would totally be down to pick up a Valve Jr and mod it out in the future. They seem to be scarce down here too, strangely enough. I guess people just aren't giving them up.
For now, I was just looking to pick up a little something retro that didn't exactly look like an amp (and could fly in under the gf radar). I think the Greta wouldn't be so great for all-around use. Still looks cool, and I'd still snatch one up if it was used and cheap enough.