The one clue I found in your description of the problem is that the lowest bass is present and "smoother, without distortion" yet the midbass was missing. I have seen exactly this problem before when the subwoofer was mis-timed, and out-of-phase with the speakers. This cause a significant cancellation of the frequencies around the crossover point, which would be smack-dab in the wheelhouse of the mid-bass. Here is a measurement of my own system with the subs both mis-timed and properly timed:
You can easily see in the cyan trace that there is a wide, deep dip from 63 to 150 Hz. This is over 1 octave wide and the deepest part of the dip is over 20 dB down from the average. I can tell you that this sounds like the bass is completely "missing." This occurs because the subwoofer drivers are moving outward while the speaker woofer drivers are moving inward. The simultaneous positive/negative deflections causes a complete cancellation of the sound. By re-timing the driver movements so they are in-phase with each other, (sub drivers and speaker woofer drivers both moving inward and outward at the same time), there is no cancellation and that yields the flatter response seen in the green trace.
I did this simply by changing the subwoofer "Distance" setting the receiver. The cyan trace has the subwoofer Distance at 10.6 ft. and the green trace has the subwoofer Distance at 14.8 ft. The Distance setting is actually a "delay" setting and by lengthening the Distance of the subs, you are effectively delaying the speakers longer so the waves from both sets of drivers arrive at the same time, (in-phase with each other).
In your case, what could cause the two subs to need different Distance/Delay settings? There is likely different latency in each sub amplifier. One sub may do more "processing" of the signal, using different DSP modes, or using analog Low Pass Filters/High Pass Filters, etc. Processing takes time, (a few milliseconds), enough time that the arrival times at the listening position are off by a few milliseconds. That is enough time to make the subs and speakers out-of-phase. No matter what's causing it, it can be corrected by re-timing the subs using the Subwoofer Distance setting in the receiver. I suggest you start by adding 3 or 4 feet to the Monolith sub Distance. See if that improves the bass. If it does, then keep playing with more or less Distance until you have it optimized as best as possible to your ear.
Better yet, get some measuring equipment and measure your response to get it as optimal as possible.
I hope the solution is as simple as this one setting.