Jonathan, you're right... no bass management with my C-J pre. However, the ML BalancedForce Subs have custom downloadable crossover filters. I'm running my ML Expressions full range, with dual (stereo) BF 210 subs mainly to 1) partially overcome a bass null at ~50-60 Hz, and 2) to further pressurize the large (open floor plan) room.
By definition, that arrangement results in extra bass, as there is no crossover involved before the feed to the Expressions, so it is outputting down to 20Hz (or wherever its natural roll-off is). That the BF210's are running a custom filter just means they might blend-in better to the Expressions at the top end of their range (probably around 80 to 90Hz), but from there on down, both the subs and the Expressions are reproducing the same signal, therefore 'double bass'. Now, as you state, you want/need the extra output to deal with room modes and preferences (we all like the low-end a bit 'hot').
The other challenge is that there is no time-alignment between the subs and the Expressions, so they could be either fighting or overly-reinforcing each other at certain frequencies with very large nulls or peaks based on distance from each other and room placement. When setting up my mid-bass modules, differences in timing made measurable differences in the group-dealy plots, down to the faction of a millisecond.
Of note, the C-J Pre is phase inverting, so line level output to the subs is off 180 degrees vs. the Expressions. Although I can (and have) manually adjusted the BF 210's phase, I'm thinking the line-level approach might synergize better.
Adjusting that control definetly can help, but it is a fairly coarse thing, measurements can help validate this. Also, use the line-level input that uses the crossover (in other words, not the LFE input) in the sub. Set the crossover to whatever works best in-room.
Line level should be the preferred connection, as you are feeding a signal that is NOT being 'modulated' by the ESL speaker. When feeding the speaker level as the sub input, you are getting a signal that reflects the interaction between the power amp and the ESL.
The ideal would be to run an active crossover between the CJ preamp and the power amp and the subs. Preferably, one that allows for time alignment of two independent sub-outs, which implies a 2-in, 4-out DSP-based unit.
The "double bass" issue will only arise if I simultaneously feed them LFE from my old Denon home theater receiver, while also feeding bass via speaker (or line) level signal via the C-J Pre Home Theater Bypass. In that situation, I should be able to compensate (via Audyssey MultEQ or manually) in my receiver. Thus far, however, I haven't even hooked up LFE, as I primarily use the system for two channel listening. That will likely change now, as I just upgraded to a 77" LG OLED!
Over the next couple weeks, I'll play around with XTZ Room Analyzer measurements, and listening tests, and see what works best!
You should be able to avoid the Double-Bass situation when using the Denon, as during set-up, you'd configure the Left/Right as 'Small', tell it you have a sub, and the Audyssey setup should be able to adjust delays and perform EQ so the blend of subs to mains is optimized for the room. The feed from the Denon subwoofer out should go into the 'LFE' inputs on the subs.
But when listening to content via the Denon (through the CJ HT Bypass), you definitely do NOT want to split the CJ line-level L/R outputs to the subs, so you need a way to toggle that.
But my advice is just to get a Marantz AV8805 and be done with it. That is one nice preamp / HT processor, super clean and even handles time-aligning 2 subs. Handles all modern 4K formats (for that lovely new LG). I have its predecessor, the 8802A, and love it. Only a Trinnov would be a step-up.
