Replacing op-amps and the end result is highly subjective and you have to know exactly what kind of sound profile you want. Some create a more warm, others colder, others still a more neutral (with many variations on these themes) profile that highlights (or hides) certain things. You can literally spend years find the 'perfect' set for YOUR ears....that may out and out sound like garbage for 'my' ears....and when you replace your cans...you might have to start all over again. It can get THAT exacting. I recommend a trip over to
Head-Fi.org - Headphone forums and reviews for audiophiles. THEY can describe the pros and cons way better than I can and give you oodles of options. Honestly, start with the stock option (they are decent) and spend your cash on GOOD cans. A good set of headphones are a lifetime investment. Just be warned if you do decide to go down that road...it can become very additive.