This is an ancient thread, but I'm about to do the same thing to my Arvin 9.
It works "ok" with some guitars plugged in, but next to no volume with others. which I suspect is due to an impedance mismatch. When I feed the same guitars through an fx box and then into the Arvin, they all sound great, so my plan is to put one of those inline jack preamp units into the radio so as to power it by the radio's batteries.
Also, anyone know of a way to switch off the radio functionality when using it as an amp?
I'll answer this as I just put the pre-amp in the radio with great results.
The radio, though it has 6 batteries seems to operate on 4.5 volts, so I hacked the tone (music/voice) switch and used it as a dedicated pre-amp on off switch... voice is off, music is on. Then the 1/4 inch output jack on the preamp was yanked off and the tip connection on the board was wired to one of the volume pot lugs on the radio. Then, I wired the piezo rod input on the pre-amp to the 1/4 inch jack I had previously installed in the radio. The pre-amp came with tone and volume dials that I stuck (with the included double sided tape), to the inside of the radio so they're easily accessible through the back panel flap.
My CBG's now have plenty of volume and it makes a fantastic little portable practice amp. Since it picks up radio stations as well, it's multipurpose. This particular radio even has a light switch. Power outages have never been so entertaining.
That's sounds really great!...I myself had had several Arvins and they do hack both easy and sound great!...but none had a light!!!! now that's pretty cool!