You are most welcome! A very important difference between the two devices is processing power; '2x4 HD' therefore allows for more flexibility and better results with, e.g., MSO generated filters [following, a screenshot from
miniDSP's MSO tutorial for LFE channel]:
As a side note,
only the '2x4 HD' could be upgraded with Dirac Live [stereo] - if required for different use, one day. However, you can always apply any [high-quality] room EQ on top of an integration with MSO (!): automatically - like Dirac Live; or with further manual tuning.
[>
Manually time-aligning subwoofer(s) to mains - how to < might come in handy at some point, depending on your integration.]
Most probably, a decent external DAC would perform better than your AVR. However, I wouldn't expect differences to be (clearly) audible - too much masking is happening on the grand scheme of things. I would suggest you first get your subwoofers' integration fixed and your room acoustics [you actually would require to do it the opposite way around] - the latter, as much as you can commonly agree to within your household.
I would leave all the extras for future investments, as they certainly would not improve anything as drastically as what you could reach by tuning the aforementioned two categories.
GIK Acoustics has great - working(!) - products and they are still reasonably priced. In the US, I would highly recommend
RealTraps as well, as they offer more refined builds and their customer support is excellent. // REW measurements are not necessarily required, though, as you will not be aiming for studio-like reverberation times in your living room. Anyway, I think their service comes for free or at least is included with an order - so, it certainly won't hurt.
Actually, that came to my mind when I saw your pictures.
You would include the whole area of your [physical] room; which in your case includes the dining area. Sound waves require [hard] surfaces to be reflected (simplified), which includes bass frequencies. Albeit, acoustic principles are different below and above the [individual]
Schroeder frequency of any room. No walls would mean no room modes -
the best scenario in the bass region! Hence my previous suggestion to put as many
properly placed bass traps in your room, as possible. On another note, you would
never be able to make your room sound "dead" in the MF/HF region(s), simply due to the open dining space of yours.
This being said, there is no option to calculate for optimum placement of subwoofers in your room. The only way to do things properly is described
here + maybe, you should include aesthetics in your decision(s) [the current placements look fine to me^^].
NB: frequency response will change due to different excitation of room modes [and combined SPL will increase], as soon as both subwoofers reproduce the same frequencies at the same time - as intended with a common set-up MONO channel. Subwoofer size depends on the output [low distortion, max. SPL] you seek to reach at your listening position(s). Once your dual subwoofers have been properly integrated you can decide, if you wish for even more impact [through bigger drivers and/or vented or passive radiating designs]. I doubt you would feel the imminent urge to upgrade...