matching sub and amp

A Fi BTL 15 with double the power you have now in a good ported enclosure should be louder than the JL 13w7. Guessing (+3 to +6db)By general rule of thumb. But quality would not be as good. The Hifonics amp you suggested as a replacement to the JL 1000.1 you have now would barely give you any added output than you get from your current amp. The numbers from the hifonics amp very over rated. Thus you would most likely not even be able to distinguish the difference becuase you would not be getting "double" the power. Thus just wasting money IMO. I would add another JL 1000.1 and a BTL 15. If you are wanting to truly gain anything that could be heard by ear. Also with that much current draw you are looking at a high output alt and 0 gauge wire.

What other amp (less pricy than the jl) would you recommend?

Well to buy an amp that is not inflated on numbers you are going to haft to pay more. Plain and simple. You do get what you pay for in quality. For a true 2000watt RMS amp new. You are looking from $400 to $800 even off the net. Rockford Fosgate t20001bd would get it done. Ebay you can get them for just under $600 new