As long as there is plenty of current, the TPA6120 works fine with one volt below its spec'd minimum (this is where actually testing the device -- measuring distortion, etc -- instead of just going off the datasheet pays off.)
About your comment "I am not going to publish all details about the modification, it isn't necessary for me." It would be to your benefit to do so, as other DIYers from player.ru already have -- as I noted earlier, you seem to have only repeated their work. Though choice of THS opamp and the PCB layout seems to be your own ...topologically, I bet it's absolutely identical to Symmcat/Storb . ...you even have the same flying electro caps. HINT: if you're going to borrow a DIYer's non-profit design, and charge $130 for it, don't also steal the dead-giveaway external appearance... like here:
Oh ... and speaking of the mod price ... $130.00 ... wow! that's steep. I orig purchased both my T51 and CK4 brand new for about the same price. And many newer/better models have since hit the Asian market ... for about the same $$.
Edited by alphaman - 9/27/13 at 10:40pm