hello matthias,
i have a dbx 164 which i think is a stereo version of the dbx 163 (but not 163x) which i think is much newer with inst di input.
I have the schematic for it but its currently packed away (hiding somewhere in 40 storage boxes) as im renovating my house. If you can wait a few weeks i'll post it as soon as i unpack my stuff.
I opened the 164 and found 2 seperate pcb's for each channel, there were even two power transformers!
i believe the 163/4 are essentially an unbalanced and no vu meter, preset version of the 165 comp. There are heaps of trimpots so maybe they can be brought to the frontpanel??
will scan and post asap. The 164 manual is interesting too so i can also post this if anyone wants.