A little elaboration?

It weighs more and will not hold-up as well being loaded in and out of places (i.e., bigger chance for breakage versus multiply plywood.)

 

That's because it's cheap. It's a perfectly good material for speakers as long as you never have to move them.

 

MDF (not particle board) is more sonically desireable because it is more inert and less prone to resonance than plywood or solid wood. I leave MDF scraps outdoors all year round, and they do not absorb water and turn into oatmeal. Particle board behaves that way.

MDF at 49 pounds per cubic foot is heavier than Baltic Birch at 39.84 pounds. Cheaper plywood is even lighter per cubic foot. These weights are calculated by beam-scale measurements of my current load of Baltic Birch and MDF. Properly braced MDF cabinets are transportable without the joints coming apart. They are a pain to transport because of the weight.

MDF is the easiest to machine for no splinters and accurate cuts. The dust is probably carcinogenic, so count on being dead within a hundred years.

MDF sheets are much easier to haul in a standard pickup truck. My buddy hauls my 60x60" Baltic Birch in his full size Dodge because they BARELY fits his truck, and no way fit my Toyota mini pickup. Transporting sheets is a big consideration.

The American alternative to Baltic Birch is Appleply which comes in standard 4x8 sheets. It is available here, but priced beyond the moon. Baltic Birch is MUCH less expensive here in Sacramento, and the same quality. A pity.

 

Hi,
I built a pair of cabs recently, and went for marine grade plywood.
MDF is too heavy - and deadens sound. You'll end up with a very flat/dead tone that has a very sterile sound to it. It will never warm to the sound, the way plywood will. If you're a tube amp kinda guy, i'd stay away from MDF. Go to a music shop and choose the best cab in the shop for sound/tone alone - I bet its plywood!
Construction plywood and any other cheap alternative like particle board will end up with buzzing anomalies at particular frequencies due to gaps etc. in the board, and this will gradually get worse. Trust me on this - don't cheap out on materials, cos all that time spent making them SUCKS when they sound crap at the end.

If marine grade plywood is too expensive for ya, try out the construction grade plywood with MARINE glue. The joins are very small and tight, and the vibration won't give it anything it can't handle. Be very particular about the sheets of wood you select too (warps, gaps, knots etc.)
This stuff will last through years of punishment, just be careful on the quality of sheeting.

Oh, and one more thing.........put wheels on it!
PM me if you're after more info.......

Good Luck,
smo