Yep, tubes. The easiest way to determine for sure whether it's the tubes that are not putting out equal "tube rush" (which seems to be unavoidable to some extent) is to simply switch them from the left to right channels (making sure to move only one pair at a time) and see whether the offending noise moves with them. If so, there's you culprit. Various folks are available for purchase of "DAC grade" mached tube pairs that will be more likley to be both quieter and more evenly matched. I've experimented with a number of NOS tubes in my preamp, and, although they seem to all start out quieter, they all seem to end up acquiring that signature tube rush after a bit. (By comparison, the tubes that I have put in my CD player have all remained dead silent, so I am not sure whether it is a product of the preamp or the tubes I have put in it). In any event, it's relatively simple to indentify the noisey bits and then you're off on the glorious road to tube rolling. Enjoy.