If it is fact that only the Denons have a problem with the left channel and not with the Shures, I would double check the cable on the Denon with another amp/source.

I use my XM5 daily and I also get minor noise/static. My simple "polishing" fix for this it turning the volume from the lowest to past half way and back at least 20 times quickly while the amp is powered off. After doing this the noise is gone but I do this about every other day. It won't hurt to try this...at least I hope not.lol

I've done this to my XM5 dozens of times and the problem has never gotten worse. My noise problem has never been severe but it's irritating when I hear even the slightest noise while adjusting volume. I believe many brands of portable amps can have this problem, depending on the quality of the pot.

I've read about this problem here before and my primitive fix was mentioned. I know there's a much better technical explanation than my ramblings but I don't build amps, I just listen.

Good luck.