I strongly advise you go to a local shop or dealer to give you proper advice when making your buying decision. Also, if you're going to get subs off your friend, you need to know what is available to you. You cant just look at the wattage rating, you have to factor in things such as impedance etc. Know what subs you can get, then you would be able to get an amp that's compatible. If you dont, then more than likely you'll be one of those guys who has a horrible sounding system and doesnt know why or how to fix it or even worse, doesnt know it sounds horrible....Seek advice from a pro before you make any purchases....Also, dont install it yourself...Let a pro do it...

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smpl_gy running a smpl system...
one amp, one sub and 1.6 kilowatts of 50hz and below...