I never used the ART as a mic preamp just cracked it open and started modifying so I don’t know about microphone problems. On the box I could not buy the parts for what it costs.

Where I am at right now with the modification.
As I posted before I converted both tube stages to common cathode with a transistor buffer connected to there plates. On the + 20db gain switch it is a double throw double pole that is tied common across the two switch sections. I cut the traces on the PCB between the two sides re ran the broken trace to the wiper and used the other side to connect just one common cathode stage to the output in low gain and both common cathode stages in series in high gain. I made the bypass cap on the first common cathode 220nf for brightness the second stage has a 10uf bypass. I am adding 500pf from the plate to ground on the second common cathode to lower hiss from the preamp I have 30pF on the first common cathode I may increase that also.
I found the VU meter gives you a good indication of the amplititude of the output signal at zero it is approx 1 volt RMS. So if you do not want a signal going into the amplifier connected to the box to cause overdrive in it keep it at or below zero on the meter.
I changed the tube to a Groove Tube so it is a Sovetek 12AX7 saw a post where they tried different tubes in an overdrive pedal and found the Sovetek to be best. I did try an ElectroHarmonix, a 12AT7 and a 12AZ7 and the Sovetek sounded best to me.
I will post some pictures of the guts when I get a chance I may not show the specific cuts and jumpers. Behind the PCB the tube is mounted to there is a gap of 0.4 inches for new components. I don’t see enough room for a footswitch in the stock box. If you took out the XLR connectors blanked off there holes you may have room for a footswitch at the top of the box. You could also build an external signal bypass footswitch or maybe add a relay in the box for signal bypass.

If I get a chance I may try to change the compressor output limit circuit it just kills the highs.