I know it's late in the process, but I find it hard to believe that a name-brand three-head deck has only mono in and out. Are those RCA jacks, with only one in and out? If so, you're right, it's mono. But if there are mini-phone (1/8") jacks, I suspect they're stereo. Try a known stereo tape and a cord with 1/8" stereo plug to whatever adapters are needed at the remote end to find out. Or you might try coming out of the headphone jack, which would almost certainly be stereo, with a 1/4" stereo plug.

R.

"My primary musical instrument is the personal computer."