Best Sub Under $175

I think that’s the key, 'sophisticated buyer', someone familiar with the playground.  For others it's hard to gauge what they expect if its realistic in relation to thier budget. 

You can sell 12" MTX 4500's all day where I live for 90 a pop.  People buy two of those and a amp to roughly match em.  But you could kill yourself trying to talk them into a 12" 7500.  Tell them it has a magnet 4 times bigger then the 4500, a voice coil 50% larger, 50% more excursion and all this translates into better SQ and more SPL.  Its like they don’t believe it. Like its some math equation they cannot solve. "No... no all my friends have 2 subwoofers, you need 2 subwoofers to get allot of bass." 

Later they come back talking about how low so and so's system hits, how hard it hits, how its only one sub.   Yeah.. That usually happens to be the guy that actually bought the single 7500 (only 30$ more then 2-4500's!), and powered that mofo near the rms with a quality amp... The setup I tried to talk them into in the first place.

I think I'm leading this thread off topic.. right.  Maybe not thought, all this talk of amplifier power and buying tips may be just what he needed, if he's still reading.  I cant say I disagree with anything in your "buying guide" but that it is truly for a buyer with a knowledge of products and performance in the car audio world.  To me the amp and sub are a package; one should complementary to the other and the package be  should choosen in a manner that mates well with the overall theme of the audio system.

You want SQ and are considering a 4th order band pass enclosure?  yikes.  Stick with the sealed, or ported (i would prefer sealed to ported in this case).  Down firing usually sounds good in a quad-cab truck.  You could be really creative and put Plexiglas on the side facing the passengers and show off the subs, mount the amp in the middle in a recessed area, and make a faceplate to put on and only show the amp, and not the wires (making the amp stick out just a little... that would be sweet...

Anyhow, http://www.crutchfield.com/S-44RDWUdwMQR/cgi-bin/ProdView.asp?g=130&id=features_and_specs&i=158XMDS16P  The real deal on that amp is 500w rms@4 ohms and 900w rms@ 2ohms.. with 14.4 volts of input... you'll be lucky to get much above 12.8 or 13, so its a bit overrated.  Despite that it should be adequate for those subs, you did well for 350$ those Kappa Perfects are nice subwoofers.