This is all sounding a little odd to me, guys.. Let me make sure I'm hearing you correctly.
The tube is located in the DAC section - so it's effectively on "Full volume" being fed into the SS amp. Adjusting the volume actually doesn't involve the tube, at all.
Are you suggesting that the frequency response of the solid state amp section is being drastically altered by adjusting the volume knob?
If so, your unit is faulty. And I'd be VERY interested to see exactly what is causing that to happen.