its always good to have headroom for a couple reasons. Unless you have a regulated amp like JL's your amp will never always make its rated power could make more could make less depends on your electrical system. Also your gain is never turned all the way up so you'll never be asking the amp for a full draw or full power. The reason the "more rms the lourder and clearer the sub is" is due to the fact you're actually putting closer to the power its needs since you have overhead. Example, a 1200rms amp only playing at lets say 25% is only giving the sub 300rms but a 1800rms amp playing at 25% is giving it 450rms...something to think on.
it is better to have lets say a 12 that takes 1000 rms vs. one that takes 500.
its all about the design and build of the sub put a real crappy sub like an xplode on 1000 rms and then one like a treo or summin on 500 rms the treo will more than likly kill the xplode dont pick a sub on its rms rating
You can blow a sub by melting a VC with too much power or clipping (thermal, power) or by clipping and making the motion of the sub non-linear (which will result in physical damage). You can't tell how loud a sub will be based on thermal ratings (RMS). The box, sub, and the environment it's in all play a part. So every install will be different.