It was a relatively inexpensive Harbor Freight unit. The least expensive floor-standing model they had, and it was 20% off at the time. About $200, IIRC. Anyway, it came with a few spare stone nozzles, and I put the smallest one in it.
However, you make a good point - they DO open up over time, so I suspect after all this time, I might see some improvement if I put in a new, small-opening nozzle again.