I ran the Alpine PDX 4.100 under my drivers seat paired with Polk co-axials in the rear and Polk components up front. sound has always been weak and flat. Also channel four has always had a tweeter squeal, not terrible but a quite noise you hear when no music is playing. It drove me batty enough I covered that tweeter with electrical tape to muffle the sound. I have this paired with a jl 250/1 for my sub sitting under the passenger seat.
I had been saving up for a JL HD amp to replace the alpine. But then the JL XD 400/4 was release. It is rated at 75x4 at 4 ohms. It is 25% smaller than the Alpine PDX and oh so nice. I hooked it up. Power is overwhelming. My flat music is full and crisp. And no noise at all.
To wrap it up, I will never buy another alpine amp again. If you are trying to fit something under your seat I suggest you look into these xd amps. They are hands down a better product than the alpine pdx.