Depends. If there's something wrong with the sound of the Technics, the DAC may well help. (Of course, it's also possible that there are some bad DACs out there.) At the same time, using an outboard DAC introduces the risk of jitter, which is unlikely to be a major factor in a single-deck system.
Here's a suggestion: Since your Arcam is a fine unit, try doing a blind comparison of the Arcam and the Technics. See how often you can guess which one is playing, based on sound alone. If you have trouble distinguishing between the two (and don't be surprised if you do), then it's unlikely that a DAC will do anything for you.