you might need to flat out replace some faders - AH boards have a problem with cheap Alps faders, and they are tough to find.
I have replaced them with panasonics from digikey using a spacer and different knobs because the shaft is different.
other than that, you probably just want to check the PSU and maybe some of the scary looking electro caps in the summing section. Maybe opamps there as well.
AH stuff is kinda not really manufactured that well - there is a lot of variation between channels, so its not really worth going in to each channel.
Also, another popular mod is to redo the groundinig on the entire board. Remove the ground from the ribbon cables, and ground each channel individually to one point on the chassis where the star is. I think that will take a lot of noise out of the design.