Can anyone help with Consonance M100 valve amp problem?

Hi all,

I have a valve amplifier that I really like the sound of, the Consonance M100. I have owned it for around 10 years and in that time it has been used on and off, in no way heavily at all.

Recently the problem I have noticed is that the sound coming through now seems to sound like I have an EQ on it, like having boosted the mids and high-mids. First thing I thought was maybe it's time for a tube change, but before that I wanted to confirm it wasn't just me so I took the amp to a friends place who previously owned the same model amp and he also confirmed the same issue on his set of speakers.
Now the amp takes 4x EL34 2x 6SN7 and 2x 12AT7 tubes, so I started by purchasing a new quad matched set of EL34's, popped them in and... no change. So I purchased a matched pair of 12AT7's and... no change. Having come that far I then purchased a matched pair of 6SN7's and as expected... no change.
The sound issue is on both channels left and right equally which is why I was originally skeptical of a valve problem. So after a full new set of tubes there is no change, so my question to any savvy tube amp owners is what could be causing this problem and have you ever had a similar issue?
Any help or suggestions would be greatly appreciated!