The 8033S II is designed to handle two subs. Each Anti-Mode independently measures and EQ's what ever is attached to it and that's going be dependent on each subs location.
Run Anti-Mode for one set of subs first. Second, run the second Anti-Mode for the second set of subs. Third, run the EQ program in the Pre-Pro.
It reads as if you have a ground loop as opposed to your subwoofer cable causing the problem. My guess, there's a dissimilarity in the ground where the subwoofer is plugged in and where the subwoofer cable connects to either the Pre-Pro or the Anti-Mode. My understanding, anything under twenty-five feet and there's no benefit XLR vs RCA connectors.
I ran three subs off of one 8033S II.
For your subs, I cannot recommend using Anti-Mode as it cuts off everything below 20Hz. Below are two graphs, one with Anti-Mode 8033S II and one with Audyssey XT32.
W/Anti-Mode 8033S II, no smoothing applied:
W/Audyssey XT32, no smoothing applied:
Because of this information, I can no longer recommend Anti-Mode 8033S II. All I can respectfully suggest, sell the Anti-Mode units, sell your Pre-Pro and amplifiers and buy a XT32/SubEQ HT capable AVR.
Us? I boxed up the Anti-Mode 8033S II, the Marantz SR5007, upgraded the subs and bought a Denon AVR4520CI what has dual independent subwoofer Pre-Outs and is XT32/SubEQ HT equipped and took our listening experience to a whole new level. As to cost, I charged everything and like most, I'm making monthly payments on the CC.
As to lowering the impedance, that's not going be a problem. If you don't know already, independently set the gain on the subs and take SPL measurements at the MLP where the measuring microphone is going be placed. At the MLP, with AVR provided pink noise going through the one sub while the other sub is turned off, turn the gain up until you're getting a close to consistent 75dB. Do the same with the other subwoofer. Now you're ready to run Anti-Mode.
Does this make sense? and will it keep the subwoofers from cancelling each other out null wise?
That's going be dependent on subwoofer placement.
I could not believe that Anti-Mode was so restrictive. Again, in the beginning as much as I loved the benefit of Anti-Mode, from personal experience, I cannot recommend Anti-Mode. Hope the above experience helps with your questions.