So, here are my mods so far:
Ch1: IC1 (pre), IC2 (driver), short R8, R23
Ch2: IC3 IC4 , short R39, R58
Ch3: IC5 IC6 , short R65, R68
Ch4: IC7 IC8 , short R93, R112
I replaced all the chips with AD8512. (And I notice a difference from my previous chips, it is more "transparent"...) Busman uses a different chip which I have not (yet) tried, but for me the AD8512 is pretty good.
The result off this is *much reduced* noise floor on all channels (try a dynamic mic like Shure SM57/58 to see the difference). But it disables the Line in on Ch3/4. Ch1/2 work for both mic and line.
Enjoy...
Richard