Is there a channel switching amp out there that does ...?

[pine]

An 18 watt Marshall Model 1974 gain channel and a Blackface Fender clean channel?

[hasserl]

That would be hard to do. The 1974X is a one gain stage preamp with just a volume and tone control into a long tail pair phase inverter into a pair of cathode biased EL84 power tubes, w/ no negative feedback. The whole reason that works is because the EL84's are overdriven so easily. That is one of the few amps that really work the power tubes, and where power tube distortion is the main source of distortion.

BF Fender amps use a similar phase inverter, but the power amp is very different, fixed bias octal power tubes and using lots of negative feedback.

If you ran the 1974X preamp into a BF Fender power amp it wouldn't drive the power tubes hard enough and would likely sound pretty dull. If you ran a BF Fender preamp into a 1974X power amp it wouldn't be very clean. It might sound pretty cool though, but probably not what you're looking for.