Hi D R,,
It is possible to get within 30 yards of a fox, I've done it. It's not easy but it can be done.
As for the Disco,,, I had one in .22 cal and after doing quite a bit of shooting and chrony testing with the rifle I would not hesitate to take a head shot on a fox because I know it has the power to take the animal out cleanly with proper shot placement. There have been quite a few GTA hunters who've taken out large raccoons with springers that are less powerful than the Disco, myself included, so taking out a fox with a Disco is certainly possible.
However, that being said,,, each airgunner needs to decide for themselves what they feel comfortable hunting with their airguns based on their experience with said airguns.
If I choose to go after a fox with a Disco that fine... If you feel the Disco is not sufficient for the job and choose not to that's fine also.
Jeff