I picked up a Soundstream Audio stealth amp. It is slightly smaller than an Alpine PDX and about half the price! I have to say, I have it pushing an Alpine Type R 10" sub in my Wrangler and I am very impressed. These little amps come in several varieties, they are stackable (but come in 2 sizes so be careful), and even pushed on a long ride didn't overheat. This amp can be installed in many different locations, due to size (9.6" x 6.5") and the fact the heat sinks are on the sides.
The problems I did have...the crossover bottom is at 40hz (not 30hz as a lot of mono/sub amps go down to) and an issue about there being no fuse to save you from poor install or shock therapy.
Check it out: