BOTH boxes ONLY treat frequency resonances between 20hz and 500hz but you have to dig a bit to find this out about the ERGO
. Additionally the fact that neither box goes beyond this range makes a lot of sense to me considering you've should have already begun to treat the missing range with acoustic treatments (provided you've done so correctly). Neither is a 'fix' and we all know that, but either can go the extra mile to help 'tame' resonances for which treatment is super expensive and bulky (ie sub 100hz).
The JBL MSC1 is a much better value. It allows you to set delay times, adjust sub polarity, has dedicated sub management and output, attenuate individual outputs, apply a hi and/or low shelf on top of the RMC, and seems more of a work horse all around. Plus its cheaper.
Unfortunately the KRK ERGO
is completely or mostly lacking in all of these areas. If you have taken care of the acoustics as best you can and are looking for a little extra resonance management or a monitor processor, the JBL is the way to go IMO. I spoke to KRK engineers about this and they are now aware of the short comings when compared to the JBL, I reckon their R&D team will address these things in the future if they want to remain competitive.