Here's the deal: The speaker cabinet's baffle board (what the speakers attach to) is like the top on an acoustic. Marshall, Fender and many other mainstream brands use quite cheap ply (or worse) for their cabs and baffles. Don't take my word for it - Check it out for yourself. MDF and particle board makes for a poorly contructed cab and the sound will suffer too. A well made cab will make an incredible difference to the tone, but you get what you pay for.

The cure used Crate 4x12 cabs, which had EVM-12L's fitted to them and the weight of the driver would repeatedly cause the screws to "de-thread" the wood. The drivers would often just fall out in the middle of an arena show. Not good. You should look for the following in any cab (no matter what brand):

Baltic Birch Ply - Void Free (Void free is very important, IMHO)
Metal hardware
Seperate baffle board and grille assembly (you can tell it's not seperate if the grille cloth is stuck to the baffle board)
Good quality vinyl (Carpet covering tends to look a bit crap after a short time)
Good quality drivers (Be aware that almost all Celestions are now made in China and do NOT sound as good as the ones made in the UK)
Check the badge on the front. Why? Because often on cheaper cabs (and some expensive ones too) the badge starts to vibrate at higher volumes and it sounds awful.

If you have a 100W tube head, then you need to make sure that you get a cab that can handle it. A 100W head will peak at way over it's rating. Thankfully, all but the very cheapest cabs will have plenty of power handling with 4 drivers in them. Celestion Vintage 30's are my favourites (they are actually not 30W, but 60W - Except for some of the Custom stuff still made in the UK which is rated at 70W)

I could go on for hours about it, and if you have specific questions then let me know, but make sure you A/B the cabs in the store. Trust your ears and you won't go far wrong. If you can, try and get the cabs together in an isolated room so that you can hear if there are any hardware rattles etc.

Since this is an old thread you probably already made your choice; Hope it worked out well for you.