I actually have to thank you for reminding me and that I am pleased to say that I am back to putting something together, though you'll be slightly shocked to hear that one of my TC's forgot it's settings the other day. This doesn't bode well as I need 100% reliability in some of the work I do.
Ok, HPF - using a full band compressor as an example. - The threshold of a full band compressor is 'crossed' by the largest area of signal passing through the circuit. For us bassists, that's pretty much always the low end. One of the downsides to this is that the top end can end up being masked as the low end is pulled down in signal (the top end goes with it). If you are driving excessive low end (especially sub sonic frequencies) in to your compressor then these frequencies could be overwhelming the threshold. Or to put it another way, sounds you can't hear are actually limiting the output volume of the ones you do want to hear.
* I'll add this before I get jumped on by the compressor police: Some full band compressors use a side-chain filter to remove low end in the sensing circuit to alleviate this problem. Multiband compressors do not suffer either, but in either case, removing frequencies that are not going to be of any use in the signal chain isn't such a bad idea. Think I'm off to buy an HPF..lol