Problem solved:
View attachment 4153931
I've got a little cleaning up/reorganizing to do, but in case you can't tell from the messy pic, it's:
A bunch of stuff > SSDI > SSDI line out to "E.W.S. Subtle Volume Control" > RFE > Messenger
Glorious, thunderous tone, and it's not going to melt my Messenger. I happened to have the EWS on hand because I used it back in the day to tame the output of an incredibly hot Fender guitar amp. It's just a passive 25 Kohm pot in a box, but it does the job swimmingly. Josh recommended 50kohms, and I'm not smart enough to know the difference or why it would matter, but he said the 25 would work fine, and it seems to. I can totally crank the SSDI and then lower the output with the EWS, and it seems to have no negative impact on my tone.
Thanks for the tips/advice, everyone.