Amp Size

2000W max or RMS?

2000 watts RMS = 2000 watts RMS.  2000 watts PEAK = who knows.  So, as long as RMS ratings are given, simply match them.  If your sub claims it can handle 2000 watts RMS, purchase a 2000 watt RMS amp.  If RMS values are not given, the equipment is a piece of crap don't buy it.

Audiobahn claims all their ratings are given as RMS.  I can tell you from personal testing that this is not always true.  But, since it is what you have to go by, then certainly go by their published data.

Yes, what you are asking is a very important question and one sure to generate controversy, but in general there are two things to remember:  1) it is very rare that anyone except SPL competition cars actually run equipment at max output, and 2) RMS ratings are the top average output at rated distortion levels, and generally leave 20% to 30% head room available (until you hit "peak") for transients.  So, if you use RMS ratings (assuming they are accurate) and set up your equipment properly, you should have plenty of head room without exceeding the equipment's capabilities.

Using larger amps to gain headroom is a technique best left to the pros, since it places severe operational limits on your equipment in terms of how you set it up and what operating bands are safe.  Remember too that it is a myth that a gain control will limit an amplifier's power output.  It does not.  Any amplifier is designed to produce full rated output no matter the position of the gain control.  Gain (actually input sensitivity) is simply used to match the amp's input stage to the voltage being generated by whatever is driving it.