I think you're just complicating what's already solved in the RME device.

The ADI-2 DAC FS has the necessary headphone amplifier. It is fully capable of driving DCA E3 out of the box. Adding an external headphone amplifier will do nothing in this case.

The RME headphone output can be used in any case, regardless of the external headphone amplifier that will be connected to the XLR / TRS outputs. Each output (including XLR / TRS) of the ADI-2 DAC has individual settings including EQ / Width / Crossfeed / Volume / Mute. So you can make corrections even with an external headphone amplifier.

When using an external headphone amplifier, the ADI-2 DAC output should be kept at optimal (best N+D) settings. The A90D in low gain mode can accept input signals up to 9.1V RMS = +21.4 dBU. I would suggest setting the ADI-2's XLR outputs to a fixed level of +19 dBU (@ 0 dBFS) and feeding it into the A90D's input in low gain mode. Then adjust the headphone volume using the A90D.

No, I wouldn't recommend it. It already has one of the most versatile headphone amplifiers built into it. I have had experience listening to DCA headphones through the ADI-2 headphone outputs and have not noticed any problems or the need to use any other amplifier.

It‘s a bit a matter of taste, but I tend to use ADI-2 for volume control, it‘s more precise, and Auto Reference Level takes care for proper gain when using EQ or other DSP functions.

Here‘s my setup guide for both variants:
https://forum.rme-audio.de/viewtopic.ph … 21#p161721

Two different approaches to use ADI-2 with external amps, how to set them up:

CASE 1, analog volume control:

To maximize ADI-2's dynamic range (S/N ratio) it's best to have ADI-2’s volume control at 0 dB, using the external ANALOG AMP’s volume control to set listening level:

• Set external amp's volume control to middle position or some above.
• Switch OFF ADI-2's "Auto Reference Level".
• Set ADI-2's volume control to 0 dB.
• Select the reference level that fits best to your loudness demands.
• Use the ext. amp's volume dial to control the final loudness.

• Once you start to use any of ADI-2's DSP functions like e.g. the EQ:
- Compensate possible level boosts with ADI-2's volume control until the Level Meter does not show overs.
- Or switch back On “Auto Reference Level" and let ADI-2 calculate the best value.

CASE 2, digital volume control:

If you want to use ADI-2's digital volume control to set listening level, ADI-2 DAC / (Pro) has a clever option that increases the usable dynamic range by 18 / (20) dB: "Auto Reference Level":

• Switch ON ADI-2's "Auto Reference Level".
• Set ADI-2 DAC's (Pro's) volume control to -18 / (-20) dBr.  (1)
• Set ext. amp's volume control (or passive attenuators) slightly above your normal listening level.
• Use ADI-2's volume control to set the final listening level.

Now ADI-2 automatically adapts the reference level for best fit to your loudness demands, while maximizing it's DAC's dynamic range.

If your external amp does not have a volume control, use variable passive attenuators to fit the level:
https://forum.rme-audio.de/viewtopic.ph … 42#p120442

SIDEMARK:
(1)
The volume figures change from dB to dBr when Auto Reference Level is “ON”.
The dBr relates to the highest Reference Level then.
There is some control range and digital headroom above, that is reserved for intersample peaks or weak digital signals.

With “Auto Reference Level “OFF” the dB-figures (without r) relate to the manually choosen reference level.

Not at all, as long as they make proper contact.
Neutrik, Switchcraft or Rean plugs are reliable and don’t cost a fortune.

Musician’s stores are a good source for affordable quality cables, like here, 6.80 bucks:
https://www.thomann.de/de/pro_snake_tpm_25.htm

E3 (?) you mean USB-cables?
No problems if they aren’t too long, say 2-3 m.
5 can be critical, 10 and more works with so called “active” USB cables.

I use a lot of different cables on different headphones - I can’t hear any difference.