What you're looking for has been done , if only partially , by Valco back in the 60's. This unit was manufactured for various brand names (Gretsch included) and referred to as a reverbration amplifier. A split output is taken from the guitar amp and fed to the guitar amp speaker and the input of this reverbration amp. The load is then reduced by a resistor network and fed through the reverb transformer through the tank and on to the reverb amp. The reverb portion comes out the reverb amp speaker and mixed with the original guitar speaker. This is a cool principle and , having had one of the original Gretsch units, I can attest to the fact that it worked really well and sounded great. I build an amp from this circuit now just by cutting out the reverb portion of the circuit and using the first tube as an input. It sounds really creamy and it doesnt matter what you play through it. I demo it with a Squire Tele and people are amazed. Here is the circuit:
http://www.webphix.com/schematic heaven/www.schematicheaven.com/bargainbin/valco_510-121.pdf
http://www.webphix.com/schematic heaven/www.schematicheaven.com/bargainbin/valco_510-121.pdf
