Bought the Musical Paradise MP-301 MK3 tube amp and although it sounds great I'm having constant 'bzzzzzzz' sound coming from the left channel. The buzzing goes away only with very low or max/near max volume levels.   What I've tried:   - Changing tubes between the stock Chinese ones and my...

Any recommendations regarding the best gain settings for Sennheiser HD598? The standard settings are 2.5x/6.5x. Are they good or should I use custom ones?   Thanks in advance!