1u 4-endpoint streamer / DSP / DAC?

Anyone know of any projects to build something like this based on Pi or something else:
[The goal would be to have 1u of 4 x 2ch of flexible audio processing to pair with a 2u 8ch amp (e.g. Buckeye) as part of a housewide multiroom setup that could be controlled from a phone or a media server of some sort (e.g. Roon)]

Internet > ETH > [[Some device presenting 4 streaming endpoints (Airplay 2, Chromecast)]] > DSP processing > DAC > 8 channels analog out > Amp > speakers

?

There's the Arylic M400 4-Zone Preamp that comes pretty close, but it doesn't look like you could load any DSP / room correction software onto the device.

Even cheaper than that (& more capable?) would be to put 4 Arylic Up2Streams into a chassis, but I'm still wondering how to do DSP before the DAC.

I'm guessing there are reasons others aren't solving this problem, so please let me know if this is a silly question.

Thanks!
Jeff
 
@thirdicomplex - I love the MiniDSP lineup, but I'd need 4 of those for what I have in mind, 1 for each room/zone/streaming endpoint. And, it doesn't have any streaming capabilities, that would need to be in a separate product that would need ethernet input, a bunch of extra external cabling, etc.

Here's another example of something closer to what I'd like - the Bluesound B400S / NAD CI580 V2, but a) it lacks options for room correction prior to internal DAC, b) overpriced, c) ... not DIY!