XLR/RCA dilemma

I would Scrap the Velodyne and get a REL 10". I have 2 Velodynes on other systems in the house still, and the REL hooked directly to the speaker terminals with their proprietary direct speaker wire connections to the terminals not in use from my McIntosh Mc452 is so much better in every way makes me hate any Velodyne, EXCEPT the Mini V 8 inch, that thing is not bad at all. Just check out a REL, it's out of this world! https://rel.net/shop/subwoofers/serie-ti/t9i/  Get one for a deal in a day with overnight shipping here: at Amazon https://www.amazon.com/REL-Acoustics-T-9i-Subwoofer-Lightweight/dp/B01ICA01U8/ref=sr_1_1?s=electroni... for the money, you cannot beat IMHO. 

Sorry, I haven't read thru this thread, but (unless there's something in there I don't know), I'd go single ended into the SMS-1 and single ended out into your amp.  The SMS is a PITA to optimize, but it's really, really good.  The improvement in overall SQ is tremendous - it's amazing what happens when you get the bottom two or three octaves right.   I'd imagine it's even more dramatic with little guys like your Kefs serving as the mains.

As to signal integrity purists, the SMS involves ADC, signal processing in the digital domain, and DAC.  Why sweat the balanced "philosophical" issue?

caveat:. If you have noise issues and/or long cable runs, you might have a decision to make.  Otherwise, I'd just go with RCAs.