I say it is a bad match of speakers.
There is no good reason for using any other setting of a 4PRO than MONO........... EVERY other possible use will usually sound anemic and quiet, unless amazing care is taken.
The Mono rating is for 4 ohms, so 2 810E are a no-no for mono.
Better to go mono into one speaker cab. BUT the 810E, IIRC has the flat-back speakers, NOT the ones from the HLF cabs..... Could be wrong, I don't have the BOMs any more....
You can EASILY over-power the flat-back speakers, even in a 810, with the 4PRO. If you do, you will know it from the very loud "CRACK" noise as the coils hit the magnet.
Anyway, I think ONE 810 of suitable capability will be louder and better sounding, more fun to play, than two run one per channel. In "stereo" (dual mono), you get less power, and your only hope is that the "wall-o-sound" of 16 10" speakers makes up for it. Depending on what you want, it might......... but......
For some reason, I have never really heard a good loud result from even 4 speakers run dual mono..... Something about headroom just plain rocks......