The stock op-amps on the STX are fairly neutral, changing op-amps is better answered on the thread listed below.
You can get 3 LME49860NAs on eBay for $15, it's a low cost way of getting in to op-amps (operational amplifiers).
Also on eBay someone has the LME49990NAs for $18.50, pre-mounted ready to install.
(The LME49990NAs have to be mounted in pairs to a SOIC to DIP-8 adapter).
Also the AD797 are well liked, they as have to be mounted in pairs (two per adapter), you can get them off ebay (pre-mounted)
But they ship from China, land of the fake, relabeled stuff, but the prices are good.
Anyway, basicly, you need to remove 4 small philipshead screwes on the Essence STX to take off the shield, then pop out the op-amps and put in the new ones.
Check out the "Essence STX Q/A Tweeking Impression Thread" on Head-Fi.