Very interesting John. Those 2/12 cabs are great , I have one and I love it.
I also have one of their 4/12 cabs and when you get the chance to crank it, that cab rocks. Funny but you get two very different responses, even with the amp set the same.
You say you keep that effects loop choked down, I keep that wide open running through a studio Lexicon.
Mesa's are funny because the more you play with them, even just between channel and master you get a different sound. I find that they sound great only when you drive the main tubes,(el34s or 6l6s, not sure if thats right.) and the main 5U4GBs.
I know what your saying, my first guitar amp was an old WEBCOR tape recorder turned to input output playing out of 2 inch speakers.
Cool John, you keep rockin'.
Glenny Trouble J

Don't argue with a stupid person. They will pull you down to their level... And beat you with experience..."Samuel Langhorne Clemens"