Too much boom

I have a pair of speakers that are boomy, sometimes slightly mushy, and at other times nice rich bass. They lack clearness in the highs. The tweeters have been replaced at some stage. Probably blew them. These are a cheaper range of speakers and probably cheapened all down the line.
Fleetwood SR88.3 speakers

Internal speaker box size 9” x 9” x 36”

125 tweeter
150mm mid
220mm woofer
220mm woofer
I was thinking of enlarging the boxes by a third, changing the shape so that all sides are different, adding bracing, and an additional port. Would this be the way to go?
I did a simple search on "Fleetwood speakers" & found out they are 'marketed' out of the back of some flunkies van or car.....the pitch goes "We have an overstock of these speakers & the Boss said we need to move them NOW". There is no information who builds these....It's all on the sly.
As it turns out....I'd bet they are ultra-cheap on build quality, Chinese made (No China slam here!).......The drivers are probably all mis-matched & any cross-overs inside completely wrong. However.......You may have something here which can be reworked......perhaps changing key drivers......reinforcing the enclosure....or ???? Can you take some pictures of them? Knock on the sides of them as if your knocking on someones they sound hollow, do they sound solid? Often times "they" build very poor cabinets so not as to spend much money creating them.

When you knock on them they do have a decidely hollow ring and they sound as if they are not dampened.
The problem I have with passing them on is knowing someone else is going to be disappointed. I would like to start out using these speakers to gain some knowledge whilst modifying them in the process.
So already, I need to modify the cabinets using a hardwood ply, battens to firm up, and damping material.
Any other suggestions?
The reason these "white van" speakers are so terrible is not because they're so cheap, but because they are "designed" using an ad-hoc engineering process. They just slap any cheap stuff together in a random process. Even cheap drivers can be optimised to produce acceptable sound with limited frequency range and dynamic response. Some commercial manufacturers (like Bose) employ this method. (The "acoustimass" system is a glaring exception; there is no way to make it sound even barely acceptable. )
Not So Bad

Hi, I know this is an old thread but it's the only thing I could find on these speakers, so thought I'd put in my 2 cents worth in case anyone is thinking of getting some second hand. I was given a pair of these and was going to put them in the man cave, but after listening to them they are now my main living room speakers (replacing $600 Goodman Imagio IC-120s).
To my ears they have a better sound up and down the frequency range, and more bass, having twin 8" rather than the 6" of the Goodmans. I can now turn off my subwoofer that was needed with the Goodmans.
An audiophile used to $10,000 speakers will probably say these are rubbish, all I'm saying is they sound better than the Goodmans. and if the OP's don't sound too good it's probably because they've been thrashed and modded,
Now my ears are old and not the best and you should not take my word for this, just saying if you have a chance to pick up some cheap have a listen to the speakers and not to people who have not heard them.