Amp/Speakers ?

[J4CKO]

Just updated my head unit, almost complete but am thinking I could do with a bit more power, not drive past and loose bowels type power, just a bit of additional power to start with, my car has the standard Mercedes system, not the Harman Kardon one so it doesnt have a separate amp, the HK ones did and added two additional speakers on the rear deck/parcel shelf so was thinking so 6 by 9 speakers there, I dont want to drill the car to bits and want to keep it looking standard.

Dont want to spend much on what is something I dont need to do, just abit more clarity and power, woudl that work with leaving the rest of the speakers powered by the head unit and then running an amp for the rears ?

if so, how much would I need to spend, would probably buy SH to be honest, loads of unwanted car hi fi out there.

How much power do you need to run 6 by 9's ? what are good brands, what to avoid, I had some 30 quid boss sound ones which were cheapies in an Audi once and they made a hell of a difference.