Well I am done cutting up the ART for a tube overdrive for now. What I have is I cut the traces on the bottom side of the PCB to free up pins 6(plate) 7 (control grid) and 8(cathode). I have removed R51, C40and R61 from the PCB. I installed a 2k cathode resistor from pin 8 to pin 9 (heater ground) a 300k resistor from pin 6 to the junction of C10+ and R37 the plate supply and a 20k resistor from pin 7 to ground (grid resistor). I installed a KSP44 transistor with the base connected to pin 1 the plate of the first triode with a 10k emitter resistor and the collector tied to the +48 volt supply. A MSPA29 instead of the KSP44 might be a better choice you can use any NPN transistor that will allow 60 volts from emitter to collector. At the emitter of the KSP44 I have a 100nF coupling capacitor with 100k resistor in series to the control grid pin 7 of the second triode. The emitter is also connected to the low gain position of the normal/+20db switch on the PCB. I cut free one side of the normal/+20db switch to select either one triode in low gain or both in series for high gain. The wiper of the gain switch hooked to the triode outputs is connected through a 10k resistor to the base of Q4. The collector of Q4 is connected to the +48 volt supply where it comes into the PCB. The other contact on the gain switch is connected to the plate (pin 6) of the triode. On both triodes I have connected a 150pF cap to ground to reduce hiss (high frequencies) when the gain is near maximum. Cathode bypass cap C34 was changed to 220nF for brightness. I changed the stock China 12AX7 to a Sovetek, Groove Tube it sounded better over driven plus may have had feedback with the stock tube. If you wanted to add a brightness control you could increase the value of R64 at the input to the preamp and install a pot with a capacitor in series across the new resistor value.
Here are pictures of the mods
http://i1020.photobucket.com/albums/...nARTstudio.jpg
http://i1020.photobucket.com/albums/...nARTstudio.jpg
and schematic of the triode section
http://i1020.photobucket.com/albums/...strevision.jpg

From what I have measured you have approx 0.4 inches between the bottom of the PCB and the cover so room enough for the new components. To play it safe I have put plastic tape on the inside of the bottom of the cabinet. I may put RTV on some of the parts when I am all done.
In the future I may try to change the compressor circuit it just kills the highs, maybe just replacing R77 with a capacitor of the right value will do the trick.

Will have to get some help to post a sound clip of the results in the future.

I would E-mail ART for a schematic to see what is what.