A point to be aware of, if you would be connecting the SP20 to the amps via balanced connections: According to John Atkinson's measurements
the SP20's balanced output impedance rises to 3460 ohms at 20 Hz (while being about 730 ohms at mid and high frequencies). The balanced inputs of the MC275mkVI are specified as having input impedances of 20K. Since the two channels are connected in parallel when that amp is used in mono mode (which is not the same as being bridged, btw, to make a minor correction to what was said above), I suspect that the balanced input impedance in that mode would be only 10K, which is not an ideal match relative to the measured preamp output impedance. For that matter, 20K isn't completely ideal either, being less than a 10:1 ratio.
Also, if you are driving your JL sub from the preamp's RCA outputs, and if (as is often the case) the preamp's RCA and XLR outputs are not independently buffered (i.e., driven from separate output stages), the sub's line-level input impedance of only 10K will worsen that situation considerably, and result in significant deep bass rolloff of the signals seen by both the sub and the main speakers. And JA's indication that the preamp's unbalanced output impedance is half of its balanced output impedance is suggestive of the possibility that the RCA and XLR outputs are not independently driven.
So before making this purchase, and assuming that I am correct in envisioning that you would have balanced connections to the amp inputs and unbalanced connections to the sub, I suggest that you ask McIntosh what the balanced input impedance is in mono mode, and ask ARC if the preamp's RCA and XLR outputs are driven by separate and independent output stages. If the answers are as I've speculated above, I would recommend against proceeding down this path.
Very nice system and setup, btw! Good luck. Regards,