I have a Marshall DSL40C, that I use as my home practice amp.
I pulled it apart and put in a new Celestion Creamback speaker, a larger choke, and new Tung-Sol power tubes.
The thing is a small(ish) beast. Way more clean power than I need in my basement, but it sounds amazing.
Then, I got my Helix a couple months ago...
I originally had it plugged into the front of the amp... sounded like crap. So, I went out and bought an EV ZLX-12P.
I'll admit, the EV monitor sounds really good with the Helix... but it doesn't have that brute power that I missed with the tube amp.
This morning, I decided to try plugging the Helix into the Return line of the Marshall amp.
This effectively cuts-out the Marshall's preamp, and just goes straight into the tube power amp section.
HO - LY - CRAP! It sounds friggen amazing!
I'll admit, it's not FRFR. I won't be running vocals through it.
But for guitar, it sounds incredible. Much better than the E-V powered monitor (in my opinion)
For anyone who has a guitar amp, with send/return on the back, I'd recommend giving this a try.
I know this isn't a new idea... but I just wanted to relay my experience and feedback.