Just got an M4 with the cathode bias mod yesterday. I did a quick sound test with an Egnater SL2X, a JF Twox and Custom RVerb, a Salvation TwinFace and MarkUs, a Sacred Groove SL+ and a stock Grail.
The Fender (stock) setting has the big bass and sounds great with the clean Twox mod, while the Marshall and boutique settings are more mid-focused. I really like the feel on the boutique setting with the mid-focused mods, the RVerb and MarkUs will live on this setting (the RVerb has a 3-way tight switch as well.)
I assumed the TwinFace would live on the Fender setting like the Twox but found dialing the Deluxe side in on the boutique setting was super cool with my Tele bridge. The SL2X also sounded cool on boutique but the JF and Salvation mods were a clear cut above. The Sacred Groove SL+ and stock Grail weren't as dramatically affected so I didn't spend much time re-dialing those between settings.
Next up is shooting out the stock Randall RM4 with the modded Egnater M4, so far I'm planning on leaving the RM4 with the stock Fender value and leaving the M4 set to boutique as I haven't preferred the Marshall setting on anything.