In a sealed box, the front panel acts as a resonator that traps/cancels it's tuned frequency. The resonant frequency is dependent on the volume of air space in the box, and the thickness of the front panel. Being rather frequency specific and narrow bandwidth, the resonator traps are good if you have measured/calculated to know what frequencies you're going after. If you're not sure, they can do more harm than good, though it's pretty safe to say that smaller rooms will need a lot of help in the low frequency area.
The pegboard faced box will be sort of like a Helmholtz resonator (a box with slats or ports tuned to very specific frequencies), a panel resonator, and a porous absorber (that traps sound waves and converts them into heat). These with their wider bandwidth, are not as effective at specific frequencies as the resonators, but are a good general treatment that covers a wide range. I like to put a curved (poly cylindrical) face on these to add diffusion to the equation, while also blending the "box" into the wall nicely. Here's a couple of
poly diffusers that I made (mainly for diffusion, not bass control) for the control room.
PLEASE note, that I'm just interpreting stuff that I've read and been told, and am by no means an expert in this area. In other words, I just might be talking out my arse (among friends though).