I would put a filter sock on, but a clean one, not the old one. Mechanical filtration IMO solves a lot more problems than it usually creates. ORP is really complicated and affected by so many inputs. I would guess that removing the nasty old filter sock probably removed a lot of reducing agents (rotting organic material) and thus created a temporary spike in ORP. It will probably stabilize pretty rapidly. Except in the case of people running ozone, or the tank becomming contaminated with a strong oxidizing agent (like chlorine bleach, though there are probably others) I've never heard of anyone ending up with too high an ORP. So unless it passes 500 and/or your livestock starts stressing, I wouldn't worry too much.
Also keep in mind, that this measurement may not be constant throughout your system. If it is measuring a bit high at the output of your protein skimmer it is probably less to worry about than if it is measuring high in the deeper and/or lower flow sections of your main tank. Are you sure your main display tank is reading this high throughout? Are you sure your ORP monitor is not having issues that would cause it to give you a false reading? If you're still concerned, you might run a thorough series of water tests to double check for any other chemistry issues that may be related, or try dialing back your skimmer a smidgen, or turning down your pumps for a few hours. But I wouldn't panic just yet, odds are the tank will right itself given a little time.
