So I was thinking of using the 303 as a guitar effect through my effects loop on my tube amp. I'm sure it would work, but does anyone know if it would damage anything? I'm about 1000 percent positive it would be fine, but I want to make sure before I do it from anyone with any experience or knowledge on the situation.

You shouldn't have any problem at all. I've done that with my SP-404SX and it sounded good.

Awesome to hear! I knew it would work, but testing it on something that costs this much is for sure something I don't want to do. Thanks a lot!

No problem. Better safe than sorry if you're not sure. The mic input works just the same with guitars since both of them have no preamp. You can also run line level instruments as well. Just keep the gain knob turned down all the way. I run my x0xb0x through that input as well. Sounds nice with the tape echo.

Good news. I still have to get a cable to finish my plan, but I'm wanting split my signal (highs and lows through a stereo eq) and add the 303's tremolo to the highs. I don't know how it will sound, but it might be fun either way. I didn't even think of it not really having a pre-amp.