I'm a little confused... if whatever's on there was recorded with DBX engaged, it will sound like strange compressed mud if you play it back without DBX. If you switch DBX on to play back a recording that was made without, it will also sound strange and possibly horrible. Don't judge the DBX until you're sure you're using it right- the difference between the sound quality (not noise floor) of recordings made with and without DBX is extremely subtle, and is more a matter of 'slightly different' than better or worse- some like the color it imparts.
I and some others prefer not to use it on slightly wider-format tape (1/2" 8 track for instance) but many very experienced people use DBX with excellent results, and I'd imagine it's somewhat of a necessity on 1/2" 16 track. DBX works by compressing the signal before it's recorded and then expanding on playback... you can't just switch it on or off after the material is recorded.