I am one of those who had humming noise with 6SN7 new tubes on Darkvoice 336SE headphone amp. After reading many threads including suggestions for Mod I had hard time to believe Darkvoice would design and manufacture any amp that would not work well with driver tube (6SN7). I didn't feel like modifying the amp even it's simple to do. I preferred finding a solution that not requiring modification because I believed that any modification to the amp is going to change the design intend of the product and it could also lead to sound signature change or else.
I contacted Darkvoice and they provided me with a simple suggestion: Aging the new tube.
By following their instruction my 6SN7 tubes are now quiet and sounding really sweet.
The aging process time varies depending on how loud the humming noise you have on individual tube. One of my 6SN7 took 120 continued burn-in hours to get rid of the humming noise completely. Below is what I suggest you to try before doing any mod to your amp:
1. There are total of two stock tubes in Darkvoice 336SE Headphone AMP (back tube: 6N5P | front tube: 6N8P).
2. Remove back tube-6N5P( leave the socket empty).
3. Keep 6SN7 tube (the new tube that has humming noise) in its socket.
4. Turn on the Headphone AMP and leave it on continuously for 48 hours (for burn-in the new 6SN7 tube).
5. After 48 hours put the back tube 6N5P back to its socket and listen to it.
6. Continue burn-in process If humming noise is still persist, until it's gone.