I received my MX5 today and had some issues and managed to fix it, which might perhaps point somebody into hopefully the right direction.
There was no speaker output, just occasional 'pops', not loud, of some medium frequency. This occurred with analog and digital input. Headphone output was fine.
I upgraded the firmware, which was 1.02, to 1.04 and the problem remained, but the frequency of the pops grew to about 1 per second.
These pops seem to be in line with some issues I've been reading online.
On a different pair of speakers everything seemed fine!
So I started looking at my speaker filter and dug up some old forum thread. My woofer has a small capacitor over the main woofer inductor, which helps for higher frequency cone breakup. It had no resistor in series with this capacitor. I've read this could tax the amplifier in a way it might not like. Other crossovers of the same manufacturer DO have a resistor in series with the capacitor!
I had a few 6.8 Ohm 10 W resistors laying around so I soldered them in series with the little capacitor, and now everything seems fine.
There are no pops, no hisses, and I can't detect any noise when the speakers are muted.
Since my other two amplifiers, all simple class D or T type jobbies, worked fine with these speakers, maybe the MX5 is sensitive to inductive loading or whatever happens when this resistor was missing.
Of course each speaker is different, but it might point into the direction of some form of sensitivity that this amp has to certain speakers and filters.