Nothing special about them. Just the name "Pro" helps to sell them. They are nice as far as prebuilt boxes go, but nothing special.
I don't like the newer ones, they seem to have a more coarse coating than before and some subs don't like to seal against the baffles. Alpine subs stand out as not sealing well on them, thier cutout diameter is somewhat smaller than most subs out there, so they just barely mount in ProBox enclosures.
Probox 'SuperBass' boxes are tuned in the 42-45hz range. Loud as hell in a sound room, but lack lows in vehicle. I by far prefer AudioEnhancer to ProBox. Every pair of 12" subs I have pulled out of a 'SuperBass' box and mounted in an AudioEnhancer KPVR box sounded FAR better.
The coating on the boxes just helps w/ moisture, but if your trunk is leaking enough that you NEED a bedlined box, you should spend $ on fixing the trunk.
Also, at the MSRP pricing of a probox, hell...I build custom boxes for less.
If you needed a decent looking sealed prefab for a specific vehicle, they do the job... All the dealers around here are selling their inventory and leaving em alone... Probox only wants to sell the enclosures loaded with their subs... The ext.cab down-fire boxes are alright. Had to shave a brace to get 2-12" CVR's motor to clear (on one side), but they sounded decent. I'd rather them over the cheap- prefabs any day.. I'm not big on the generic Probox (non vehicle-specific).. If you have that much room then get a custom built to specs..