I agree with Jay 1 as those subs mentioned would not be much of an upgrade at all as it would be more of a lateral move, The Klipsch rw12d Jay 1 has posted can be had from time to time from New Egg for $300, it seems they are on sale about every other month.
You mentioned you were pretty happy/satisfied with the Dayton 120, I assume you are looking into more "umph"... or are you looking just to fill the room more?? If youre looking to fill the room more then I would suggest running dual subs. Running dual wount really give more bass in essence but it fills the room out much more. I see you can get the Dayton sub ( now 1200) for $120... looks as its a very good price, I have never had the chance to hear the Dayton but I did at one time have dual Bic Pl-200, which is a slightly better performer than the F-12 you mentioned. THe Bics are a pretty decent sub esp. when you take in the price its just that I dont think it may be a HUGE difference from what you have already.
When I replaced my dual Pl's with the PA-150 I noticed quite the difference, as the PA-150 hits harder than the Bics every could. I now have an Outlaw EX along with the PA-150 as I can tell a difference between the two but onlly in the lower hz such as 18-30. Of corse both subs are over the Budget you listed as the PA-150 I think can still be had for around $380 as the OutlawEX is $650.
I guess my point is unless your willing to strech the budget you listed I dont think there will be much more of an inprovement IMHO. If you are just looking into filling out the room then I would just suggest going with another Dayton 1200 for $120 as that will save you money as well.