Box design is a part of it, but so is the vehicle itself. Does your friend have a hatchback? Maybe an Eclipse or Integra? That would explain a lot. Is his set-up ported? yours sealed?

Also, I think you have your sub wired in series, which is not a "wrong" way, but may not be maximizing your amplifier. If you sub is dual 2-Ohm coils, then you have it wired for a total of a 4-Ohm load. If you wire it differently, you can get a 1-Ohm load (per sub), and the lowest you can get with 2 subs is a 2-Ohm load.

Also, just because your amp "says" 1700 watts, does NOT actually mean that it is actually putting out 1700 watts. The Kenwood site lists a KAC-9103D as rated at 500x1 at 4-Ohm, 900x1 at 2-Ohm, and Over 900x1 at 1-Ohm. "Maximum output" is listed as 1800. The way you have it, you are getting maybe 500 watts. If you re-wire it, you will probably get close to 900-1000 watts. Keep in mind, this is listed at 14.4 volts....which you will probably only get with the motor running. Once you shut the motor off, you are probably closer to 12.4 volts (depending on how good/new your battery is). Also, take into account voltage drop for the wire run itself. If you are using cheap 8 AWG power wire, you might not be getting a full 14.4 volts AT the amp.

WAAAYYY too many variables without more information (at least), and possible actually seeing both set-ups...

I hope that helps a bit.