By getting an amp the speakers should sound clearer, and go louder. Basically, to a certain extent, the more power you put into them, the better there going to sound.
I would suggest anything above the 2x100WRMS mark will do fine. The brand and power rating will depend on your budget, but don't go too overboard on $$$ for a 6x9 amp.
Don't forget to add in the cost of wiring for the amp, including an inline fuse, RCA leads and power cables. There are usually wiring kits for sale in most audio shops, about $40 for the kit is the norm.
Hope this helped.
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