My LD MKIV tube amp is not playing well with my newly acquired Denon D5000 headphones. I first notice that the right channel was playing at a lower volume, wrecking the imaging. That was last night. Today I get serious distortion from the right channel. I swapped output tubes and moved the power tubes from left to right, but the problem persists.
 
Now here's the really weird thing -- my H650s get along with the amp just fine, as do my PX100s.
 
Likewise, the D5000s play clear and distortion free from my other amps, as well as directly out of my laptop and CD player.

What could possibly be going on here?
 
FWIW, I also tried different gain settings on the LDMKIV.  The amp's manual says it's rated for headphones 32 ohm and higher. The D5000s are 25 ohm. Could this be the problem? If so, why did they sound okay at first?
 
EDIT: There's definitely a problem with one of the power tubes. The problem -- lower volume and distortion -- followed the tube to the other channel when I swapped.
 
Any ideas why the Hd650 would sound relatively OK with a bad power tube while the D5000 distorts badly? Is impedance playing a role here? 
 

I feel like a bad power tube affects the pathway to the rest of the system i.e. more resistance. When the power is pushed up, the difference is not noticeable because the higher current "pushes past", whereas in the same conditions except you use a much lower power for lower demanding headphones, the current gets "caught" more on the bad part of the faulty power tube. But I'm not an electrical engineer

The bad tube is definitely the culprit, but the higher sensitivity and lower resistance of the D5000s makes it more obvious.

Tha is right the lower resistance of the D5000 makes the tube work a bit harder with more current and that tube can not keep up.

Yeah the MKIV is going to put out a LOT more power into higher impedance headphones compared to lower impedance headphones.  That is just the nature of the OTL tube design.
 
My Little-Dot II+ has similar power characteristics and it runs out of power really quick with my 24ohm XB700 headphones but has a ton of power with my 250ohm DT990-Pros.

After 2 years, finally an answer. Thanks guys!