I actually spent a bit of time looking into the current state of RPi (well, Linux/alsa) audio options. I came away disappointed/frustrated for the most part. A truly state-of-the-art multi-channel DSP system based around the RPi will likely require some custom hardware, or else just throwing in the towel and using the Okto DAC8 Pro.
As @sombodyelse points out, managing output to multiple cheap DACs is more work than it first appears. There's the MiniDSP UDIO-8 which gives 4xspdif in/out, but it was a bit underwhelming in Amir's test - rather high jitter. Plus, it's $325, and a NanoDigi is only ~$200, so if you don't need inputs you might be better off just going with a simple streamer with stereo out and implementing the DSP in the NanoDigi. Even so though, you're looking at 600-750 by the time you add 4xKTB which puts you up into the price range of the pro audio interfaces which give you even more flexibility with potentially comparable performance in a single box, and you're only a couple hundred shy of the Okto which would seem to be a no-brainer in comparison.
Diyinhk has an ESS9038 based 8-channel DAC kit using the Xmos interface which should be class compliant and work fine under Linux.
https://www.diyinhk.com/shop/audio-...hz-32bit-es9038pro-pcm-dxd-dsd-audio-dac.html This kit is only $200 with xmos interface, and so might be the leading candidate if a one-box solution with custom enclosure is a high priority. Plus, it's 'likely' that a bit of hardware work would allow interfacing one or more A/D converters to the XMOS board to provide some input functionality.
The big disappointment I discovered though is that even though the Motu interfaces are "class compliant" it seems they don't really work under Linux at this point - they "work", but quickly run into channel mapping and distortion problems. (this is the AVB / 8A / 8D range - it looks like the M2 and M4 work fine with the latest kernels). Focusrite Scarlett interfaces and the Behringer UCM units all appear to work fine, but they're a step down (or two steps in the case of Behringer) in performance from Motu. Using the MiniDSP USBStreamer to get ADAT in/out and pairing that with a Motu 8a or 8d looks like it would be workable, but it wouldn't satisfy the one box/ custom enclosure requirement.
TL/DR So, after all that it seems that I'm rather uncertain on where I'd suggest going with this idea. Options
- 4 channels with input capability the Motu M4 seems to be the best fit. Would basically allow re-creating a MiniDSP SHD aside from the lack of digital inputs and would run ~$300 including an RPi 4. I actually have all this in-house, and so may actually start tinkering.
- 8 channels 'high end' would be to use the Okto DAC 8 Pro. It offers AES/EBU inputs, but they're problematic since they have to be clock masters, and so this would make most sense if you didn't really need inputs. Truly state of the art, but a 2-box solution. figure $1300 or so
- midrange or better >4 channels with inputs - a pro audio interface. The Motu AVB interfaces are what I'd want to go with, but that would need diving in to address the driver issue with unknown prognosis - some capable people have already been looking at it without success. The Focusrite Scarlett interfaces just don't excite me but they may be the best option for 'out of the box' if you need more than 4 channels - $500-700 and again a 2 box solution
- the DIYINHK ESS 9038Pro kit is the best thing I've found so far for a more or less ground-up custom approach. This obviously assumes that their implementation is at least competent, and requires assembling the kit which may leave some people out. Base parts cost is pretty reasonable though - maybe $400 plus the enclosure.