it depends on the RMS, if the RMS is 250 then yes get a 500 watt amp or larger. On JLs it is usally safe to run to about 3/4 of the max of their subs. What is the max of the subs?
As far as the 2 channel amp, yes you need a 2 channel. the difference between a 2 channel and a 4 channel is tht a 4 channel lets you have the option to run a few more subs or your midranges, or subs and midranges. its basically so you dont have to buy another amp if you want to run everything. For your application i would tell you to get a 2 channel RF or JL 500.1 or higher. As far as Xplod amps, i have had a 1000watt 2 channel for over 2 years and its running just like brand new.
EDIT- make sure whatever amp you buy has a FULL CROSSOVER, if the amp doesnt all your doing is killing your subwoofer because its letting voices out of the speaker..