Hi there, I’ve had the M51 a few years now, and still enjoy it.
I agree with the consensus - the USB input is not the best on the M51. What I recommend instead is actually the Yellowtec PUC2 Lite which is an excellent USB-AES converter. Gives a very meaty, powerful sound. I prefer it to the iFi iLink (USB-SPDIF) which is laying around unused.
From there, you can add as many USB clean-up devices as you want. There are tons of (too many) these nowadays (Uptone, Intona, new iFi products). I still use the old iUSB and Mercury and Gemini cables.
I agree with the consensus - the USB input is not the best on the M51. What I recommend instead is actually the Yellowtec PUC2 Lite which is an excellent USB-AES converter. Gives a very meaty, powerful sound. I prefer it to the iFi iLink (USB-SPDIF) which is laying around unused.
From there, you can add as many USB clean-up devices as you want. There are tons of (too many) these nowadays (Uptone, Intona, new iFi products). I still use the old iUSB and Mercury and Gemini cables.