garage audio... :-)
i'm not sure that there is any better bang for the buck than those infinity's. there have been a few folks around here who have used them with good results.
typically when you get into really cheap subs, there is no magnet (that is what they are cutting out) and so the motor strength is low, qts is very high, and you end up with the peaked response results like you have in your picture up there.
parts express tends to be a popular place for subs built by diy folks and if you poke around there you'll see what I mean by the infinity's being such a bargain.
I wouldn't worry about temperature swings so much. drivers are pretty durable in that respect. I suggested four 12" drivers and I still think would be a good suggestion.
maybe there is a question in there that i'm not seeing/not responding too...