I didn't mean to be rude...
But when you said:
mrsmith77 wrote:... "RC4558P" and "OPA2134PA." ... it might be those labeled "MN3101" and "MN3007."
I guessed your confusion...
Improve the sound is a little vague:
if you need less hiss then swapping a mere 4558 with an OPA (or 5532, at least) is a good idea,
this "mod" won't change the sound at all, hiss aside, since op-amps are simple buffers or mixers, here.
if you need a deeper chorus, you'll have to mess with clock capacitor (C10 on this http://www.freeinfosociety.com/electronics/schemview.php?id=2290
the same if you're looking for a more flangey sound.
This "mod" will change the sound a lot, you'll have to evaluate if this is an improvement or what...
A switch on "out B" ring lug and GND will give you a preview of the "vibrato mod"
You may also trim LPFs at your taste for a more open or close sound.
All of this will be easier if you can read a schematic, of course...