Need some info... I'm a litle bit confused Options

[Buffalo]

As some of you allready know i just got my 98' convertible few days ago.

It has the 6spkr sound system (4 speakers + 2 twiters) with a kenwood head unit.
I don't know if its the factory unit or an aftermarket one but it looks OLD.

The thing is that i already found a new one to change it but i want to keep the factory speakers
and maybe add a single channel amp + 10"-12"sub.
Seaching through the FAQ's i found that my factory system allready has an amp for the speakers but some say
it can be reused with an aftermarket unit and some say it can't.

I'm not an expert but i know that if i want to keep the amp i need to use the preamplified output.

So my Q is how good the factory amp works with aftermarket units and what is the max power of the factory amp+speakers ?
what is the indivitual max power of each factory speaker?
The head unid i'm looking at has build-in 4x50w amp. Do i need new speakrs or not?

F.Y.I: The speakers and amp are in good shape and they work fine even when i pop the vol too high.

ps. sorry for the long writing. I just need to know as much as possible before i go out and spend money on things that i can't use on my car.