> a link to the schematic:
> You could put in a standard Fender EQ after the first tube
It _IS_ a "standard Fender EQ after the first tube". They just replaced the pots with fixed resistors. (And drew it badly.)
Open this large image: http://i.imgur.com/HwxwZ.gif
It shows the "Champ 600", the real Champ with Bass/Treb knobs, and the snazzy Twin with Bass/Mid/Treb knobs.
From Champ AA764 to Twin AB763, they made the 15K bottom resistor "M" adjustable to trim the mids. (It is also smaller, 0-10K instead of 15K fixed, because the Twin should never be as mid-heavy as the cheap Champ.)
Now look at the "600". Where the others have 250K Treb pot, it has 180K+75K. This is just like a 255K pot jammed at 30% resistance (about 7 on 0-10 audio taper). Where the others have 0-250K Bass rheostat, the "600" is stuck on 75K (again, like 7 on the knob you don't get). The fixed Mid resistor is taken directly from the original B/T Champ; but could as well be a 25K or 50K audio-taper pot.
Master volume just means (generally) replacing the 220K fixed in front of the 6V6 with a 250K "Volume" control, just like it has at the grid of V1b. This will reduce gain. At some point it will limit output power. Meanwhile it is wasting away what little NFB the AA764 has, so it gets less polite.
> a single webpage about this mod...
I don't know of anything specific to the "600". This is *general* Fender guitar amp design and tweaking.