Prima Luna Tube Question

Hi - I have a new PL Dialogue Premium HP amp; connecting it to my system out of the box one of the channels did not work (no sound).  I started the troubleshooting process; move the speaker connections and then swapped the gain and driver tubes left to right - in both cases the channel outage followed the left to right swap.   Finally thought I isolated the issue to one of the driver tubes.  

I replaced that tube with a new PL branded 12AU7 tube and I got sound out of the channel, but noticed it was not passing the full signal to the channel (it seemed to be missing the mid range and the stereo imaging was focusing on the other channel).  So I started the troubleshooting again by swapping the tubes across channels to see if the problem followed the tubes - it did.  I eventually got fed up with the troubleshooting and replaced both gain tubes and 2 of the driver tubes with a new set of 12AU7's.  Through this effort, the problem seemed to come and go and resolved itself temporary after resetting the tubes.  

Anyone have ideas on what the potential issue could be?  Am I focusing on replacing/re-seating the wrong tubes?

Thanks - Michael