Hello

A few weeks back I noticed that My Super Plexi was begginning to emit strange distortion on lower notes, and chords.  The sounds spewing forth are sort of a "sub-octave ring modulation" type sound, if that makes any sense.  They do NOT occur on higher pitched, single note playing.  I realize that this is a symptom of worn out power tubes, so I purchased some new ones, biased using the video instruction off of the Eurotube/JJ website, and unfortunately, am still having the same problems occur.  As a newb, where should I begin looking for problems inside me' amp.

Thanks