1. HK will have good bass but the sound is not nice and crispy. It will depend on your taste. Some people say it is ok, and some people like me hates it. So don't waste you money (unless it is free) buying that stuff. You will be wasting your time and your money.
2. For the $400 budget, it is going to be a really tight budget. The following is what a good system will cost you:
- 4 Channel Amp - Rockford Fosgate 400W ~ $250
- 2 sets of components speakers for front and rear - $100 each.
- Low Output Converter (LOC) - ~ $25
3. If you change your speakers, you will have to change the amp, and if you change your amps, you will have to change your speakers. The reason is because you will have to match the impedance of both. If you listen to music in moderate volume, then changing your headunit (around $650 for Pioneers without Nav) it will make a difference. But, if you listen to your music at high volume, you will hear some distortion. But I have heard several people say that they are satisfied just changing the head unit.