After reading this review I had to get an MK V myself. I was lucky to find a used unit which got here last week and I've been toying around with it since then.
I haven't been too lucky so far and couldn't solve some of the problems that occured to me, not even after reading the whole thread.
As I'm using some low impedance, 35 ohm Ultrasone phones I had to get an impedance adaptor to get rid of the constant hiss the amp produced. With an additional 120 ohms, this problem went away.
Two problems that still exist though seem to be related to my dedicated source which is a PC, to be more exact an Auzen Forte 7.1 sound card.
The first thing is that I can hear the mouse movement or the HDDs spinning up as soon as the amp is connected to the PC. The volume of this sound is annoying and doesn't change with any volume setting I can adjust, neither on the PC nor on the volume knob of the amp. I do not have this problem when I connect the phones directly to the card or use a different amp (a Meier Headsix in my case). Both are dead silent and give an absolutely "black" ground level.
The second problem is gain. So far I haven't been able to use the pure, un-amped line out of the card as the amp doesn't deliver enough gain to do so. Only when I use the built-in headphone amp of the card I can use the MK V to listen at a decent level.
So far I tried different interconnects between amp and card, snap-on ferrite cores, different power cables and power outlets.
All inputs the card has are already disabled, muted and set to "zero" volume. No other cables are connected to it but the one lead leading to the amp or the headphones, depending on what I'm using.
I also tried to move the USB devices around to see if there was a difference and I made sure nothing wireless would contribute to this kind of noise. When I turn off the PC the noise is gone completely, as soon as I turn it on the noise is back.
Is there anything I could use to filter the input and to increase the gain setting?