best box for SPL

This is false. Within the ports most effective area power handling is actually increased both thermally and mechanically.

You both are correct and wrong at the same time...

Mechanical power handling increases near tuning of the box (usually much more than a comparable sealed,) but one octave above or below that tuning the mechanical power handling is actually less.  Also the THERMAL power handling does NOT change.  Thermal power handling is 100% independant of the box design.  Thermal power handling, is quite simply the amount of power that a coil can dissipate.

As a point of refrence my setup:

4 '03 kicker L7 12s  (rms 750w each.)  To each of these subs I am running a Zapco 9.0.   Each of my 4 9.0s carries a rating of 2200watts RMS.  Needless to say I can go WAY over the thermal and mechanical limits of those subs at about EVERY frequency save 38-50hz  loudest note is 42hz.  Above and below that range they will bottom when I bring the full 8.8kW to bear.  And of course Im hugely overloading them thermally....

Keep in mind the more power you apply to the sub in real time, the smaller the bandwith it is capable of playing due to the mechanical limits of the sub.  On my setup I am very limited in the frequencies I can play at full volume.  At normal listening volumes I am fine....  In a sealed box you will have the best powerhandling across a very large bandwith.  A ported box has a smaller bandwith and overall its power handling is less, however near tuning it's mechanical power handling is much much more than that of a comparable sealed.

Guess i wasn't specific enough. Thanks for clearing that up though.