CONNECTING A SUB WHEN THERE IS NO SUB-OUT CONNECTION

Excellent. That is the perfect connection point. 

I’ve read it in this thread, as well as in others, that connection of a sub via the high level inputs is preferable to take advantage of the characteristics of the amplifier, but “characteristics of the amplifier” being an advantage implies to me that those characteristics are synergistic with those of the sub, placement, rest of the system etc and rather prone to luck of the combination or very specific choices. Is there an electrical advantage related to the combined loading on the preamp of the combined amp and sub amp compared to the preamp only seeing the amp and the amp seeing the low impedance speaker plus the high impedance sub?  

Really nice system. I’ve owned BAT gear in the past - not at your level but I did enjoy it. 
 

I came to realize that I’ve a second set of XLR-outs on my preamp from which I can run cables for each channel into left and right subs. 
 

As @ghdprentice said this is probably best. My entree to subs was REL and speaker level connections and I drank the cool aid. IMO this connection is great if you have no alternative. You do. XLR pre out is what I’m using with my Arendals. 
 

Is there an easy way for me cut low frequencies to my main speaker and shift the bass load off of them?

I don’t believe so. But often guys are trying to offload the amp duty rather than the speakers. You’ve got loads of high quality power driving the mains and I doubt the mono blocks are breaking a sweat so I wouldn’t worry about it. 
if you get a pair of great subs dialed in and pressurize the room properly my guess is you take some of the load of the mains by not needing drive them as hard

@macg19 wrote "Really nice system. I’ve owned BAT gear in the past - not at your level but I did enjoy it." 

RESPONSE - Everything goes on sale if you're patient.  I don't like to compromise and will wait to get what I want at a price I'm willing to pay.  I've been mostly patient--except of the DAC purchase.  One came up and I sold my a$$ on every corner to get it.  It's no longer offered.  I heard that BAT could no longer get boards supplied from Playback Designs and the Rex 3 DAC was never a commercial success even though it's likely one of the best DAC available at any price.  

@macg19 also wrote "But often guys are trying to offload the amp duty rather than the speakers. You’ve got loads of high quality power driving the mains and I doubt the mono blocks are breaking a sweat so I wouldn’t worry about it. "  

RESPONSE:   I'm using Martin Logan Dynamo 10s which have built-in high-powered amps.  Because my speakers are full-range e-stats, I'm thinking that if I can take everything below about 50hz and run those low frequencies through the subs, the panels will be happier. I'm still trying to dial it in.