What will the solution/answer be . Get a RC 
He could controle it with a pc or streamdeck too LOL
But we are still in testing fase to do it with brain waves gniffle.
Yes, RC is back for me now (but I first had to find some batteries)
no, it’s hardware volume control
I cannot exclude that this is due to the amp. It’s an old Linn Wakonda Pre Amp with an Allo Volt+ D as power amp. But in respect that it’s the same with a new headphone amp (Monoprice Monolith THX 887) I think this is rather unlikely.