you might listen to the mixed cab configuration...if you haven't already..... before you're sold on the idea.....
just validate that tonally you get the results you are after...as depending on pairing, you may get different than expected results
having said that.....
a second head slaved or run parallel, possibly using a box that eliminates back-feeding of the pre signals
or a decent PA amp, considering signal pre out/pre in match ups
Pre + PA amp setups can be finicky, so don't assume any pre + pa amp will work well.......or get driven to full power so well.....
with the ER stack, you have lots of sound (as you know), but maybe not as much of the cutting yet full tone of a good 4x10.......
if driven well, you might find that a single HS410 or AE410 might give you what you want w/o using as big a rig......
I have an ER stack and an AE410......both great...but for me...the AE410 puts it out there in a different sort of way...more audible I guess, but still has the muscle, and ample low end..... IME/IMHO
I have also used an AE410 + AE210 with 2x MB LMII, one being slaved off the other.........
It peeled the asphalt off our street.....
either way...good luck,

