Problem with headphone amp

[blindmullet]

Hey folks my amp has slowly gotten weaker in the right channel and low end distortion has started to increase. I suspect crappy Chinese tubes. Amp has about 50 hours mostly left on all the time during breakin. At about 20 hours it got really sweet sounding with excellent midrange but all frequencies were very smooth and balanced. Since then it started going the other way and now about a 10% reduction in right channel output (very seat of the pants estimate).
Tube compliment is 6N11 for preamp and 6N5P output, not familiar with either tube or weather that's the problem just need someone in the group that's more knowledgable than me to offer an opinion. Don't know if this matters but it has what has been described as a Black Voice or Dark Voice circuit design. It has a very very low noise floor and has been an excellent singer. Way better than I ever expected from a low cost Chinese amp shocking really how good it does or did sound. Phones are Sennheiser HD 598 and HD 600. Thanks for your help.