My cab was constructed strong structurally, as most cabs are, but it didn't hurt to add a few wood screws in the bracing for added strength and rigidity.
My concern with cabs, new and used, is that speakers may not be mounted right ( bolted through the baffle properly ), wiring quality is normally under grade, and they are not sealed as air tight as they could be. Ported or not, the cab corners, seals, and speakers need to be air tight for best speaker responce. As for as adding foam, that insures a better sounding bass freqs in a cab and clears everything up imo. Just keep it away from the port holes or you will blow the foam out while playing; from what I hear.
I think I know why most cabs don't normally come with such attention to detail such as being sealed really good, better wire, and foam. Good cabs are made well enough and quickly to turn a profit.
It takes a lot more time to go over every corner with a caulking gun and wait a day for it to dry, not to mention adding seals around the handles, back panel, and isolating the jack plate like I did. Speakers have their own seals so as long as they are mounted correctly then they are sealed.
Adding foam also takes time. Every part has to be measured, cut, glued, and given a day to dry.
Adding better wiring wasn't a big deal for me but the bigger gauge wire is harder to work with then the smaller stuff and cost much more. I could see this being a cost problem if I were wiring 500 cabs. I only need 10' for one cab but for 500 cabs I would need 5,000'. 10 AWG copper is $0.40/ft X 5,000 = $2000. 18 AWG aluminum is $0.12 X 5,000 = $600.
See why a company will cut little corners on stuff like proper wiring, sealing, acoustic damping ( foam ), and even speaker mounting ( not really a problem on most quality cabs ). They they will build a soild product that looks nice and really is a nice cab but it isn't worth the extra investment ( time and materals cost extra money ) if it only means a smaller profit return on a slightly better sounding product.
I most cases, the the extra investment will only cause the product to cost more then what people are willing to pay, so companies just decide to keep cost as low as possible. There are cabs that are made with all this added attention to detail, but they also carry a heavy price tag. I found it is far chaeper to just put you own time into making your own cab a little better then paying someone else labor. Most cabs are built structurally good and are just a basic shell waiting to be refined and finished on the inside. Besides, its easy stuff to do by yourself. It does take lots of time and some detail, but its not like modding a tube amp or anything.