I'm listening to my P4 speakers and N22 amp right now. I've owned it for over a year now and I'm really happy with the set-up. I'd recommend getting some kind of small computer stand to angle the speakers up so you are sitting in the sweetspot when you are listening to them. I bought a pair of the audioengine stands which work well but they're kind of pricey for what they are; this would be a good DIY project.
There are two things you can do to help out with the bass. One, as you said, is to buy a sub. I recently bought the Energy ESW M-8 sub,
Amazon.com: Energy ESW-M8 NA 1,200-Watt Subwoofer: Electronics. I love the sub and it wasn't hard blending it in with the P-4s. Plus it's small so it's really easy to find a good spot to set it up and it looks good.
The other thing you can do is to put the speakers near a boundary of some type. I originally had mine set up on a desk and pretty much out in the open. When we moved, I got a new desk with a large hutch. The speakers are inside the hutch and the enclosure made a noticeable difference in the bass. My sig line has the equipment I'm using.
