flatfoot, you might be able to match levels and impedance with the little rumble amp - however, my guess like Mac stated is that it's going to impart some kind of frequency domain shift to the signal.

You would probably be better off just buying a little 1 channel pre-amp like something from ART - something that won't put a big thing into the signal that you'll have to EQ out anyways. Speaking of EQ, if you have a hardware EQ around, you could probably make that work for your pre-amp. About the ART, It's always nice to have one of those around. You should be able to pick one up used on e-Bay for 50$ or so. I have a single channel dbx mic/instrument pre-amp that I really like that I use now and then to boost weak signals.

Lastly, do you have a little desktop mixer? You can use one of those to do your pre-amp work for you with your guitar.