OP - when you run MRC in your processor, is there a particular peaky resonance in the waterfall plots/frequency response curves?

Have you tried adjusting your speaker and/or listening position - these have a big impact on bass peaks and nulls - you're trying to find the position for both which is the best overall compromise.

Do you have any room treatments, particularly bass trapping, in the room? As you have noticed, the 818v3 is a big step up from the surround processor overall, but unfortunately you lose MRC and then have to start dealing with all these things without...

Lastly, what is your main source, as e.g., if you are running Roon, you can put the MRC filters manually into Roon's parametric EQ.

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502