I've got a MAC 1500 receiver that I just had serviced. I noticed that the left channel sounds a bit "dry" if you can call it that than the right channel. The right channel sounds fabulous. Tubey and very airy. I tried to switch the 3 small tubes and it made a little bit of an improvement. I know it's not the speakers because when I switched the speakers around, the "right" speaker sounded dry. For awhile I thought maybe my left ear was the cause but when I compare the two sides, I can definitely hear the right side sounding better, no matter on stereo or mono recordings. Could it be the tubes would need to be replaced? I believe they are the 12AU7 small ones. Any suggestions?