(1) The Quart seems okay. But, it's extremely low power handling and low efficiency indicate that it's got a very low output. Also, it's 6ohms which complicates matching an amp. You'd need 3 Quarts ($150 total) to get a 2ohm load, and I think that a single 12" Dayton DVC would still outdo them.
(2) The Advents are cheap which seems to be their only positive point. You'd probably be making a sideways movement from your current 8" subs.
(3) The problem with the RF is that it's an 8ohm sub. You'll have to get something that puts out 300w @8ohms (equivalent to 600w @4ohms) to push it. Otherwise, it would probably meet your needs.
Take a look at this
15" Dayton Thruster for $58. It is EXTREMELY efficient. You could run it with a very modest amp (easily less than $100). Let me know if this interests you at all, and I could run some enclosure calculations with WinISD.
