Stock sub are junk, they're just bolts in which doesn't do ****.
Go pick up some new MM full length subs, and a strut tower brace (for cowl shake) some decent budget shocks/struts like KYB's are cheap but effective with warranties
If you want to do expensive 'smoother' you can do some quality coilovers, better placement of what holds your car up
my car - when i first got it - rode like F150 with heavy duty suspention with no cargo in bed. hard, jittered, and generally rough. Stock Shocks/struts/spring were the reason. It rode great when your on perfect pavement. But as soon as your off of smooth pavement, it went all over the place.
I since then got Bilstein shock/strut, and Sportline, and it ride 1000000times better. Instead of above mentioned ride quality , its more like that with full weight on it.
Still gets jitter time to time. but i think thats more to do with croppy OEM control arm and fact i don't have SFC
All i've changed were the springs to eibach pro-kits and that GREATLY improved ride quality. I can't even imagine how it will feel with new struts and shocks.
I have MAC subframe connectors, MAC upper and lower rear control arms, eibach prokit shocks, struts, and springs and do have a better ride, but definately still not happy. I am saving now for the full front coil-over kit / K-member / front control arms, bilsteins, etc... Hopefully that will do it..
+1 on full length subs to replace the stock crap, also I would get rid of the UCA's and put stock units back in with OE rubber bushings if your serious about improving the ride for comfort. The LCA's should'nt effect the ride quality too much but even they should go if you want the car to be as smooth as possible. Just remember you bought a Mustang. If the ride is that important you should probably trade it on a crown vic or town car...