I'd like to use my Line 6 DT 25 amplifier (only the preamp stage) and then go to my Peavey Classic series 50/50.
If I only plug the Effect loop send, without plugin any return, does it bypasses the poweramp?
You should always have a speaker load connected to a power amp while it is running. Try running the amp in standby and see if the tap from the FX send still works. If not, you'll have to connect it to a load.