CONNECTING A SUB WHEN THERE IS NO SUB-OUT CONNECTION

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.  

I have a Rogue Audio RP-9 preamp with two sets each of XLR & RCA outputs.  I have been using one set of the XLR outputs to run two subs.  I will get my converted to stereo monobloc Odyssey Kismet amps in a few days and will be using both sets of the XLR outputs for the two stereo amps to vertically bi-amplify my Fyne speakers.  I purchased a pair of Neutrik RCA to XLR adapters to then drive the two subwoofers off of one of RP-9’s RCA outputs.

I would insert a 2 in, 4 out, balanced, active crossover of your choice, between your preamp and amps. This would enable you to achieve greater overall system performance, increased dynamic range, better clarity, cohesion and precision and to a lesser degree the ability to address some of the acoustical issues created by your room.

When I first incorporated subs into my system, some eight or nine years ago now, I started with the high level input like most everyone else. About a year or two later, just for fun, I tried an active crossover and it ended up probably being the single largest improvement to my system that I have ever made.

I purchased a JL audio cr-1 crossover which I understand is designed to work with high-end audio systems.  It's all analog, has balanced connections and, based upon my research, it’s the best available.  I should have it in a few days.

Does anyone have any experience with this unit?