Alright I received mine a few days ago and got a a couple of hours listening in. The sound quality overall is quite satisfactory and the MX5 pairs well with the SMSL Tabebuia. I mostly used Tidal Hifi over Bluetooth for convenience, but also a few hours HDTracks over USB cable with USB Audio Player Pro. Both worked fine, no issues there.
But yeah there is the hissing noise. Its not loud, but it is there. Since I have the unit in my sleeping area, I will probably wait the 30-day return period on amazon before I have a final verdict. Maybe Topping will come up with another patch in the meantime, I would expect it to be a software issue. It happens both in headphone mode and when you have auto-on (units starts upon input recognition).
Also Im rather sad they did not enable LDAC support. So if I dont keep it, I might go for PA3s and DX3Pro+ (which has LDAC, but no balanced out), even though I would prefer an all-in-one solution.
Gee, can a 300$ unit not simply do all you want, no problems?