Not convinced on that theory unfortunately. The valve is entirely bypassed unless the switch is enabled. And even if it is enabled - the Sovtek 12AX7 is used in dozens of models of amps worldwide, and I think the same problem would be apparent in those also.
My conclusion after experimenting heavily with the EQ is that the EQ 'flat' isn't really flat, the EQ controls more than rectify this and add the top end that people say it is lacking. Some people say the cabs colour the sound a lot too, I need to try this one out when I can get to one (might steal tonyfs EBS 410)