If you know what he meant, then being picky about the word he used just makes you sound like a jerk. Nobody is going to think he was suggesting buying a vinyl chessboard for an audiophile. If he'd said 'disks', no one would have though he meant a frisbee, either.
EDIT: Ah-hah, someone got to it right before me. The top paragraph is addressed to logwed, just to be clear.
Anyway, back to the original question: For $150 you're not likely to come up with anything that will actually wow him, as such - he already has equipment of that cost level or above. Your best bet is probably to get something considerate and/or special, such as a pretty headphone stand or high quality recordings of music you have reason to believe he would enjoy. You didn't mention what the turntable is, so it is possible that you could get a better one for the money, but not likely. I can't vouch for the Digizoid thingy, having never used one - although it is intriguing.
If you do get a headphone stand, remember to make sure you get one in a configuration which fits with the place he stores his headphones - a wall-mounted stand isn't any good if it needs to go on a table, and a table stand isn't any good if it's supposed to hang on a wall beside a desk. If it sounds like obvious advice, I'm just mentioning it because the first time I got a headphone stand I hadn't considered the logistics, and didn't realize that I had no practical room for a table stand
