Interface for 2.2+HP desktop audio

[arvidb]

I'm looking for one (or several?) audio interface(s) for my desktop. The use cases are as follows:

• 2.2 sound - so 4 individual outputs, at least 2 of which are balanced, either with EQ in the interface or (preferably) using CamillaDSP on the host machine. All four channels must be controlled though the same physical volume knob on the interface! (Not up/down buttons as on Topping DM7.)
• Headphone output (for Sennheiser HD 560S) - again through individual channels so that I can apply EQ specific to the HP. Preferably also with an individual volume control (but sharing with the 4 main channels is ok too).
• Doing measurements: at least one balanced input and one balanced output of "reasonable" quality for doing sanity checks of audio equipment. That is, I don't need SOTA performance, but it should be above audibly transparent with some margin.
• Linux-only environment.

As I understand it the Focusrite Scarlett 18i8 might actually pull all of this off. It apparently has 20 PCM Playback channels from the host which are fully configurable in that you can configure which PCM channels are connected to which outputs (line outs, headphones, SPDIF), and which are controlled by the (digital, so no channel imbalance!) volume control.

According to Amir's review of Sennheiser HD560S it has an impedance range of 133-224 Ω. I haven't found good data on the 18i8, but Julian Krause has tested the 18i20 and it can do 16 mW into 150 Ω with ok THD+N numbers (so 1.5 Vrms max?). Is this marginal given that the HD560S has a "sensitivity below average" (246 mV for 94 dBSPL)? (How do I convert that to dB@1V - 20×log10(1/0.246)+94?) I do like to listen loud through headphones from time to time.

The 18i8 seems fully supported on Linux (no thanks to Focusrite but to individuals who've made an heroic effort to reverse engineer the control protocol). Mainline Linux support makes it invulnerable to planned obsolescence through new Windows versions which is a big plus IMO!

Cons:

• HW/FW that is maybe a bit unstable (with the horrible failure mode that puts full amplitude square waves on all outputs!)
• Headphone outputs possibly not good enough?
• Mediocre sound quality from a measurement tool point of view?
• Size - it's comparatively huge!
• Not USB bus powered so needs an additional wall wart

Any suggestions for alternative devices that can do all of the above, possibly except for the headphone part (I have an ODAC+Objective2 HP amp that I guess I could use for that - or even drive the O2 from an 18i8 HP output)? So specifically, any other interfaces can do ≥4 channels all controlled by a single physical volume knob out there?

Or have I found my unicorn device already?

 

[arvidb]

For measurements there seems to be a very nice setup in Topping D10s + E1DA Cosmos ADC. If I get that combo I could also use the D10s as a headphone DAC feeding my Objective2 amp. Then "all" I'd need is a 4-channel, at-least-two-of-which-are-balanced, DAC with a physical volume knob for all channels.

Focusrite 18i8 is ~€400.
D10s + E1DA Cosmos (B) + USB isolator is also around €400.

I'd be prepared to pay more for the D10s+Cosmos+<unknown-DAC> combo than for the Focusrite option, but not a lot more. Say €300 budget for the ≥4ch DAC. Again, any options out there?

(Given that 2 subs + room correction is the go-to advice given here, I'm wondering what people use to make this happen. Are everyone really paying ~€800 for something like a MINIDSP FLEX? Edit: It is nice though, with the different input options and its remote control... No! Too expensive! )