The Sonic Edge FXUL8TR solved a modern problem with vintage amps, the lack of FX loops.
FXUL8TR taps audio from the 1/4-inch speaker output jack of any guitar or bass amplifier speaker and converts it to a line level signal ready to feed any external single channel effect processor. This method uses an amplifier circuit that does nothing to the sound of the amp itself and allows the amp's speaker to function normally. FXUL8TR is not a speaker soak and is not intended to replace the amp's speaker.
Once connected, the wet only output of the effect processor is fed to another guitar amplifier's effect loop return or any standalone power amp hooked to a cabinet. Create Dry/Wet (your vintage amp plus another amp) and Wet/Dry/Wet setups with a stereo return feeding two additional amp/speakers on either side of the vintage amp in the middle.
some of the other ideas....
The FXUL8TR is used to provide a buffer-isolated signal from any guitar amplifier speaker output jack.
One common purpose is to provide a line level signal that feeds an effect processor.
The wet only output of the effect can feed another amps effect loop return or power amp with speakers
to have a Dry/Wet setup or even Wet/Dry/Wet.
The FXUL8TR is also a stand alone microphone preamp with phantom power.