Stereo Help/advice

[kk93sc3]

Hey everyone.. New to the site. Just bought my brother's sc300 from him about a week ago. Love everything about it but I need to do some upgrades on the audio so I have some questions. I have the pioneer system.

I would prefer to keep the stock HU.

I already know the stock speakers are 4" so I'd like to upgrade those to 5.25" front and 6.5" in the back. I found nice adapter plates for the front ones (hopefully the speakers will fit in them) and I've read that the rear ones you can basically make fit with some bending. I'm going with Infinity kappas for front and rear (leaving the stock tweeters).

On to what I am unsure about:

I know replacing the factory amp would give me better sound but I'm not sure how to replace it. Would I have to run all new wires to the speakers and to the back of the HU? How do you go about wiring a new amp? I can't just plug the wiring that was in the factory amp into the new one, right? I know where the amp is located and how to get it out so I would rather not pay $100+ to get an amp installed.

The same goes for the sub and its amp. How would I wire that? Any idea which subs will fit without a lot of modification?

I listen to all kinds of music so I want a nice, full-range system that has refined bass (nothing heard down the street, but can hit lows).

I'm also not interested in putting subs in the trunk. I'll just stick with a free air sub.

Any advice/help would be really appreciated. Thanks!