Question Can I connect my bass to the RH450 and use the RH450's digital out to connect it to the Guitar/bass input in my USB audio interface using an XLR female to 1/4" TRS jack cable

No. The digital out requires a similar digital input (either an S/PDIF or AES/EBU input). From what you've described, the guitar/bass input sounds like a regular analog input, only designed for instrument input, so we wouldn't recommend using that with the digital output of the amp! The digital signal being output from the digital out is 96 kHz / 24 bits AES / EBU format, though you can also use it with an S/PDIF input, using the proper cable.