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it uses some old lm301a opamps...Are you sure? While the
schematic I have shows 301s, mine was built with BiFET amps like LM353.
If you really have 301 with 33pFd for the input amp, and are throwing huge drum signals into it, yes you will be slew-limited. LM353 or TL081 is the fix.
If you stick with reasonable output levels, the 301 after the VCA210 plug-in card won't be a major bottleneck because of output stage gain.
Don't diddle with the opamp on the stand-up card OA230. It has a complex bootstrap thing going on. If you don't understand it, you will smoke something. However for short signal runs, you could take an unbalanced output from the C17 cap and bypass OA230.
The only mod I've made to mine is to give a high-gain unbalanced input to simplify my minimalist mastering chain. But then I don't feed it raw drums and I only ask for -10dBV nominal output.
Speed of limiting action is NOT adjustable by mere humans. OA11 and ALL that junk above it forms an exponential capacitor. Take what DBX thought was best (or worked); or use some other limiter.