let me try to summarized the function of Red and hopefully member can understand why there's a product like Red.
First Function, streamer. Make sure you don't plug in anything in the USB in of the Red. (Else it will force the Red to function as DDC and won't boot up the sd card OS). Red need to have the SD card plug in to the slot with linux base (correct me if wrong) OS that turn a Pi base controller into a mini PC with the purpose to function as a music network endpoint. Protocol supported by it depends on the OS, e.g. Roon, sportify, NAA, UPNP, airplay etc. And it will output via USB out, coax, optical, AES, I2S. With May, since both Red and May support 1.5M in PCM to me it's no brainer to stick to USB out to May. Then you need to have a host/server where you would run music library software and send your music to Red as streamer. You can run Roon, Jriver etc. Most of these music library software come with "controller" app option, to use phone or ipad to browse your library on your host/server and play it to Red (audio network endpoint)
Some people might ask why do we need this Audio Network Endpoint while we can connect to our DAC from our PC directly. Back about 10 years ago, when one of the first streamer audio network endpoint come out (micro rendu), it seriously change the quality of computer Hifi scene. I was shock too with the elevation of sound quality with this method compare to a highly optimized PC (fanless, bare win server with audio optimizer software, linear PS etc) connect direct to the DAC via USB sitting just next to my other HIFI equipment. Then when I move my Host/server pc out of HIFI room (since network cable could run very far), I got another level of improvement. My understanding is simple, Hifi just don't mix well with digital equipment. Mainly noise get injected and radiate everywhere, via AC supply line, ground plane, over the air etc. By placing digital equipment, far away, first, the ground plane don't connect direct to each other, second, the AC line are much further to HIFI equip. Noise usually in the high freq range don't propagate far. Thus the benefit of using a simple, low noise audio network endpoint is clear!
(append: for whatever reason my ascii drawing did not show up as intended, I will amend later)
not ideal:
AC line
Circuit breaker--------------------------------------------------> Hifi equip
|__> PC and other RF noise equip
ideal: (this explain why some old school lay separate AC line for Hifi)
AC line
Circuit breaker--------------------------------------------------> Hifi equip
| |--------------------------------------------> audio network endpoint (streamer)
| | network Lan
| |
| |
| ---------------------------------------------<---
| network Lan |
_------------------------------------------------> PC and other RF noise equip (router, switch)
AC line
Second function DDC. I won't explain much on this function much. In old days, USB implementation in the DAC not that good, and couple with noisy PC connect direct to the DAC, sound very "digital". Thus DDC product come out mainly to break the noise to the DAC, reclock, output to DAC via other format like AES, coax, optical, I2S etc (which the DAC input is well implemented). Personally, I don't use any DDC as long as the USB implementation is Good and use of low noise streamer.
Above is just my experience and all are welcome to discuss in civil manner. Of course each of us have different hearing and experience, and welcome to do whatever suits you. After all the above way of optimization is way cheaper if you think about it in depth.
