best option?

[nebular18]

Hi guys, I finally made the decision to change my car's head unit to the Pioneer Premier DEH P740MP and I love it! Got a pretty good deal on it, CAD $600 plus installation and mounting kit. Looks cool, sounds great and lotsa features!

Currently, I have the non HK factory (2001 325 Sedan) sound system and the stock speakers doesn't seemed to be able to handle the "output" from the new headunit, rattles from my front speaks, muffled base and so on. Though, there is a very noticeable improvement on sound quality.

I was wondering what is my best option to improve the sound quality? I guess adding an amp (4 channel probably, is 6 necessary?) is a must, so is a subwoofer. But i am currently on a very tight budget, so it is necessary to replace the stock factory speakers to achieve reasonable sound quality that "matches" the output from the new head unit?

In addition, what brands do you guys recommend? For amps and speakers. For the speakers, do you think the Focal polyflex series is sufficient or should i do for the polyglass series? Thanks!