MM HD 210-130 distortion tone problem???

Need some help; (this might be a normal condition...). have a MM 210 HD 130; on hi-power; sounds GREAT...all original with Sylvania 6CA7's......when I flip the switch to low power; I can go about to 2-3 on the volume and start already getting some "sag"; a kind of "distortion" tone; not unpleasing; but not clean and clear; Master volume is on about 8 or so...is this normal? Are the Power tubes (which are the original Sylvania's I might add) getting a bit weak? It has the phase splitter 12ax7 and that would have nothing to do with this right?? Ideas? Again, on high power; all is well with lots of clean headroom; low power; starts to distort a bit earlier than I think it should.....

Mine does the same thing, especially with a humbucker guitar. I end up plugging my guitar in input 2 and running the volume at 1 3/4 and master at 7.
I still think it just plane sounds better in High power mode so I normally run it that way with a small fan behind the amp to cool it down