I have this 4x10, and it sounds nice and punchy in general, but the four Celestion G10l-35s are a little too biting in the mids. Depending on the amp, they can be a little harsh. Perhaps replacing two can result in a more balanced tone. Is there a sub $75 10" speaker that would complement the G10l-35's midrange aggressiveness?
If it were me, I'd put a pair of G10C's in the bottom. The Vet 10's probably wouldn't bring about as much of a change as you are looking for ... but the G10's should blow your mind. Do a search of the site (search bar in upper left) to see all the blog entries on those speakers ...
Oh, and please return here and let me know your results... I've been thinking about doing the same thing :-)
I wouldn't recommend the smooth cones ... unless you want to warm the cab quite a bit. As it is, the G10's will add quite a bit of bottom over the stock Celestions.