Re: Rythmik F8
I am running the 340s high passed at 80Hz using the bass management in my receiver. The way the L12 works, if it is fed using the LFE input the internal crossover and phase adjustable controls are bypassed. That way the receiver or pre-pro take care of xover and phase, while you can still manually adjust sub output and bass extension. In reference to the latter, I keep the setting in "Low Music", so that the -3dB point of the L12 frequency response is 17Hz.
Dave can for sure elaborate more (and much better) on the subject, however I believe that crossing at 80Hz you are already helping the CMB-170 quite a bit. Even if the F8 can be used in "stereo" mode (I doubt it), the physical setup of the sub would keep left and right so close to nullify any stereo separation. Not to mention that the spacing would be way less than the spacing of the LR mains.
I have never tried it, but I believe the best way to have a stereo sub setup is to use two physically separate subs, feed them using the "Line In", one using only the L channel, the other using only the R channel and then adjusting xover, phase and level on both manually.
Doing this and if you put the two subs underneath or very close to the CBM-170, you could have some advantage is crossing at a higher point. You would effectively turn the CBM-170 plus sub into a 3-way system, with each sub and each CM-170 being co-located and therefore eliminating any mis-localization issue having the sub reproducing frequencies up to 200Hz, because they would be emitting by the same main speakers location.
On the other hand, if you cross at 200Hz with a single sub, you would be depriving the L and R main of some of the ambient cues that contributes to correct stereo imaging in the mid-bass region, which is exactly the opposite of what you want to accomplish.
Emotiva Fusion 8100 AVR
Ascend Acoustics CMT-340 SE Main
Rythmik L12 Sub
55" LG OLED55CXAUA TV
Panasonic DP-UB420-K 4K UHD Player
Seiki SR4KP1 Region-free BD Player
Roku Premiere+ 4k