Which to keep! Pick one of these two soeakers

Just for fun, humoring me perhaps, just try running the speakers full range with one amp. First the Musical Concepts then the ARC. Listen to the 'sound' and the differences you get with these amps. I suspect that tonally speaking there might not be a huge difference, but I think that the ARC will have a smoother sound than the SS amp. I don't know what your 'rack' looks like but you can deal with the ARC's heat by either putting it on the top of the rack or placing it on the floor next to it. For sure make sure you give the amp a lot of free space around it. 

Sell that Mac and the 555. Neither, in my estimation, has the 'quality' potential of the Musical Concepts and ARC. 

@newbee Both amps sound good, the ARC sounds maybe a little smoother like you said, a little more organic while the Musical concepts is a little cleaner and in your face. Both sound really good though. One thing to consider though is that both sets of these speakers are having issues with low end when bi-wired. Each sounds much better when you push more low end to the subs. The upper registers sound much better on the Spendors, much more texture and guitars/horns sound incredible and detailed/alive. The ML are a little more piano like in the upper registers, seems like a little air is missing that is in the Spendors. I might try running the amps bi-amped, if there is a natural volume difference between the two amps maybe I can use that on the bass side to push a little more low through the speakers.

If I had your stuff and perceived problems my inclination would be to get a decent quality equalizer, like the Schiit Lokius, a six band equalizer with 2 bass frequencies, and supplement the output of the pre-amp i.e. between the pre and the amp. Costs about $300. 

BTW, are you sure that your listening position is not in a bass frequency null? If so simply moving your chair a bit could solve your bass problem. 

FWIW IMHO bi-wiring is more of a perceived benefit than a real one. I suppose with SOTA stuff and critical hearing skills the difference might be apparent, but note a lot of manufacturers have not provided for bi wiring. Some very high level stuff. The only difference when bi wiring that I could hear was when I used different cables, i.e. ones with a different set of sonic values.

To me the MLs are a no brainer. Great mids and highs and the hybrid design sounds great.  New panels.  slam dunk.