amplifier suggestion for 2000 4runner

I recently sold my Acura TL for a 2000 4runner 4x4 SR5 and I'm looking for a 3 channel amp that will fit under my front drivers seat. There are very few places in the 4runner to fit an amp. The only other location I can see is in the right rear compartment but I can't figure out how I would mount my existing amp in there (it's enormous). I only plan on running a front stage (Phoenix Gold Xenon x5.0) with one sub(MB quart 15" reference series). Head unit is a Sony CDX-GT320 and my current amp is a Kicker KX650.4 (FOR SALE!). I'm pretty broke right now so assuming I can sell KX650 for $150 or something my theoretical price limit is $250-300. Thanks for the help!

I forgot to add that I wish to preserve the cargo space in my 4runner as much as possible, after all that's the reason I bought it! This rules out mounting on the rear seats.

A lot of companys now offer a small/shallow/or stackable amplifers, Memphis S-class MTX jackhammer series and SPL gorilla series to name a few, buy a 4ch and use the ch 1/2 on front stage and bridge ch 3/4 for your subwoofer, just a suggestion.Chadillac

Yes you're right, I definitely should go with a 4 channel amp! Thanks for the suggestion. Any recommendations on how many watts would be sufficient for my set up?

As many as you can afford. 

Hey you should also look into the 5 channel amplifier with the D-class subwoofer connections? There's high medium and low end amplifiers you could try outm just google some and compare.I can't hear you!