I absolutely agree with everything you said, and I also ran across a relatively recent series of posts in another forum site where RME (MC) was talking about this exact situation.  Ordered the adapters and they should be here soon.  This will save me the trouble of having to wiggle around to connect everything. Instead, I can simply use the two RCA cables coming from the subwoofer, since the new DAC is located in the same spot as the old one.

It's not the just the quality of the converter that draws me to RME devices; it's their manuals and support that truly win me over.

I lurk around and you have been extremely helpful over the years.  Your time and patience are greatly appreciated.

Yes, indeed, the manual provided all the necessary information and more. I downloaded the manual because my eyesight is not as good as it used to be, especially with smaller print. As I was scrolling through the pages, I foolishly assumed I had reached the end when I got to the installation sections. Admittedly, I was over-excited and probably a bit overconfident, having had a similar device for over five years. I completely missed the technical reference section.

After settling down a bit and taking a more leisurely perusal of the manual, combined with some discussions found around the internet, I am up to speed now on just about everything I had questions about before.

Thanks!

The primary solution is:

• Run a pair of cables from ADI-2 outs to the sub.
Use sub’s built in low cut filter for the mains, to free the mains from large driver excursion.

• Then run cables from the sub’s dedicated main outs to the main speakers.

The cable type depends on the available connectors of mains and subs.

If your main speakers are big and full range:

• Run 2 pairs of cables from ADI-2 outs to sub and mains directly.
The mains contribute to the bass then, allowing better room coupling.
For this config XLR Y-splitters 1xXLRf to 2xXLRm (*1) are the best option if both mains and subs have balanced XLR inputs.

Instead of the XLR-Y-splitters ADI-2’s individual XLR and the TRS / RCA (depending on ADI-2 model) can be used in any combination that makes sense.

Using balanced interconnects (XLR, TRS) can avoid possible hum and noise problems, but often that’s not a problem anyway.
To take this advantage both sides, outs and ins, need to be balanced.

(*1)

I have set up two EQ profiles on the ADI DACs: one before acquiring the subwoofer and another after its installation. The subwoofer settings are stored on my phone, and I've transferred the EQ settings from the older ADI DAC to the new one. I intend to take further measurements in the future. At present, the most noticeable difference seems to be in the volume levels, which appear louder at the same reference level and loudness settings. Specifically, the ADI-2 DAC FS, when set to -54 (Auto Ref -5dBu), seemed quieter compared to the new ADI-2/4 DAC PRO SE, even though the latter indicates +1dBU. I'm uncertain if this difference is due to the accurate level being displayed when using balanced XLR connections. Previously, it was necessary to add +6dB to the reading on the older DAC when using XLR connections, a step that might not be needed with the new model.

The setup sounds fantastic, as always. I stream Apple Music through my iPad Mini, which sits perfectly atop my DAC and is magnetically secured with the smart folio cover as if they were made for each other. I opt for the highest quality lossless format available and avoid using the app's volume normalization feature across tracks. This choice leads to significant volume differences when shuffling through my extensive music library of over 50,000 tracks, ranging from modern 44.1kHz 16-bit pop songs with limited dynamic range to 192kHz 24-bit jazz tracks with a much broader dynamic range. Consequently, having volume, bass, and treble controls within easy reach on my desk is particularly beneficial.

Previously, when I used the ADI-2 DAC, I activated the Auto Reference Level feature. The display indicated a level of -5 dBu (Auto). However, since I was utilizing the XLR output to connect to my power amplifier, which in turn powered my floor speakers, the actual output level was 6 dB higher, effectively making it +1 dBu. Now, with the Pro model, I've also enabled the Auto Reference Level, and the volume display reads +1 dBu (Auto). I haven't yet connected the subwoofer. Given the variability in volume across different audio tracks and various biases, it's nearly impossible to subjectively assess the overall volume level. Nonetheless, the volume seems to be roughly the same as before, albeit slightly louder.

It seems likely that I'll need to tweak the Digital Signal Processing (DSP) settings with the subwoofer's app and the DAC's PEQ once I integrate the subwoofer. This is because the TRS output will likely differ significantly in volume if I maintain the same setup.

Thanks again! Yes, I pulled up my DAC's PEQ setting I named "Sub Connected", and then adjusted the subwoofer's main volume setting in the app from -8 to -14 and it seemed right at my regular listening experience.

-5 dBr! Nope! I like my neighbors.

I don’t know where this “information” origins from, but it’s not true.

The difference between feeding one input or two depends on the devices feed, but worst case (both destinations 10 kOhm input impedance) it’s just 0.17 dB.

For typical standard destinations (47 kOhm) it’s even less: 0.04 dB.

In both cases simply insignificant, negligible.

Plus: it‘s not related to and does NOT interfere with ADI-2‘s Auto Ref function in any way.