Unfortunately, that's not what I mean. Separating the subs alone won't solve anything. It's just necessary so that you can correct the phase for each one individually, but to do that you'll need something like a MiniDSP.
You're seeing the complexity of subwoofers, and the more there are, the more opportunity for problems. Integrating them into the room isn't easy. Unfortunately, the way it's all hooked up, it's not possible to correct it. Each sub needs to have individual phase controls at the minimum. In...
A couple quick things.
First, I'd set the LPF for LFE to its highest setting. Setting it lower limits the LFE channel, and no reason to do that.
Regarding the bass, you have a standing wave/cancellation problem. Solving that isn't easy, but it can be addressed at least. What model(s) subs...
Those, especially the LaScala might be a little overboard. LOL. If you could find a Heresy or Forte in good condition, and have room for it, that could be a good option. At least getting higher efficiency will make the balance easier to achieve.
Do you have a subwoofer? You want to be...
A couple things. Calibration is clearly a problem here. There shouldn't be that significant of a change when turning the center speaker on/off with proper surround media material, as opposed to music. Second, expecting a single, full range driver, even a relatively expensive one, to compete...
When processors have a USB input, it's usually for software updates, or just as a power source, not as an audio source. there are options for that.
My guess is, just for functionality, you'll probably end up using the AVM 60 in place of the DAC. With some changes to how stuff works.
I...