We use a HAG-USB, a Musical Fidelity Trivista DAC and Musical Fidelity A308. This afternoon, my wife stated she had heard several loud cracks from the left channel. Then I heard a loud thump too and turned everything off. An hour later, I turn things back on and the left channel is dead. Turn them off again plug in an Ipod to determine if it is an amp or DAC issue - now everything is back to normal. Any suggestions as to what is going on? I was under the impression the small tubes in the MF DAC were bomb-proof?