The problem is that people tend to set their hearts on one particular sub and do whatever it takes to make it work in what might be a not-so-optimal environment. I feel safe in assuming you want 15s because they're feckin' huge subs and 15s will get lower than 12s or 10s, right? I mean no ill will, but I hear this almost daily at work.
Here's what I always suggest:
1. Determine how much space you want to give up in your car for an enclosure and amps
2. Determine how much electrical power you can give to this system (tends to be dictated by your alternator and battery/wiring setup)
3. NOW start researching subwoofers that perform optimally in that environment
4. Find an amp that will do the amount of power required for your sub (the RMS, NOT Peak value), plus a little extra for headroom (never a good idea running amps full tilt all the time...unless you like replacing amps every few years)
That being said, budget tends to win over all (unless you have patience enough to start planning builds a YEAR out from the start). For 150-200 a sub, you've got some pretty killer options. I, like mylows and bbel, would recommend 12s over 15s in your trunk:
Incriminator Audio Lethal Injection (The I series is within your budget, but the LI is just a MUCH better sub...look for used deals to fit it in your budget)
Incriminator Audio | Performance > Hype
DC Audio Level 2 (just a hair over budget, but will take the CVR's lunch money)
dcsoundlabs.com - dc sound labs Resources and Information.
Obsidian OA12 (my first personal recommendation based on budget. No need for haggling or finding a used deal to fit it in budget)
Obsidian Car Audio
SSA DCON (KILLER budget sub...since the redesign, they do exceptionally well in both sealed and vented enclosures and about 400W per sub)
Sound Solutions Audio - Quality Car Audio Subwoofers
Sundown SD-2 (sealed) or SA (ported)
Welcome to Sundown
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Now, talking amps, I recommend one MAJOR point. Look used. Looking new will always either cripple your budget or limit your options and, in the end, you'll have to settle for less. Provided you choose well-built used gear, it will last well more than 20 years. Take a look at
eBay (for examples, not purchase) and you'll see old-school Orion, Autotek, Soundstream, Zapco, and a slew of other brands that would be WELL beyond what you could afford had you bought them new, but are now anywhere from the $100-300 range. I just noticed today an Orion 2250GX for about $400 that would THRASH any of the options mentioned (in a good way).
Long story short, a little research goes a LONG way. Sites like this,
DIYMA, MAF, CACO, and the SSA forums are literally chock-full of all the info you'll need to realize why we feel the way we do about Best Buy/Walmart brands.