[trz1000]

You need one for sure for the sub. Ideally you'd have a four channel to run the 6x9's and 3.5's and a mono amp for the sub. Assuming you use decent amps, it'll sound better that way than by using the head unit to run main speakers but if you're doing it on the cheap, all you need is one amp to run the sub.

In my Jimmy I used a Clarion PDX5, it's a 5 channel digital amp. Being a digital amp it doesn't require huge heat sinks so it's smaller and can be hidden in an area with less air flow than a solid state amp. It runs a 10" sub and two sets of 6x9's very well. However, it is pretty spendy though not too bad when comparing to buying a good 4 channel and a mono, not to mention the space savings.

I run my Jimmy topless all the time and the stereo sounds great, for a drop top. With the top on it's awesome as the bass response is greatly enhanced. It hits pretty good topless and almost hurts with the top on.

Some guys don't like 5 channel amps, they say that you can get some cross talking. I've never had any issues but I'm also not a pro audio installer.

Anyway you go, you don't have to spend a fortune to get a good sound. For great sound, be prepared to drop some bennies.