If you use a chain of multiple processes on an instrument, you might wonder what should come first, the EQ or compression. Equalisation is primarily about changing signal levels, albeit in carefully specified frequency regions, so pre‑compression EQ can alter the gain‑reduction action of the compressor, but post‑compression EQ won’t.
If you’re happy with the way your compressor is working, just put any equalisation after it in the processing chain, but if you find that frequency‑based problems make it difficult to achieve the compression you want, dealing with this problem via pre‑compression EQ makes sense.
For example: extreme low‑frequency thumps from a vocalist tapping their foot on the mic stand can play havoc with attempts to compress the vocal itself. Filtering out these low‑frequency level peaks with EQ, pre‑compression, can immediately make the compression sound much more predictable. Sporadic low‑frequency resonances from acoustic guitars or guitar/bass cabs can also be tackled in this way.