Seems like the Dayton Ultimax 15" and 18" are REALLY hard to get. Parts-Express was supposed to have them in stock again at the beginning of October, but now it seems the date has been pushed ahead to December. I went to the Parts-Express store and inquired about why they are always out of this speaker.
The salesman told me it was a supplier issue. My guess is it's such a good speaker for the price that people buy them up as soon as they get them in stock. But should it really take over 3 months to make them and ship them?
It's a shame because there's very few speakers that can match the performance of the Ultimax and if there are they cost twice as much. Dayton goes to painstaking lengths to provide T/S specifications, cabinet size measurements and frequency/impedance graphs.