How to improve headphone sound on Anthem Pre1L?

Does anyone know a way to improve the headphone sound on Anthem Pre1L Pre-Amp? I know that they use an op-amp for the headphone output. Is there a better op-amp or other mod that would help?

What's in the headphone out that you're not currently satisfied with?
What ear-speakers you're using?
Op-amp means nothing alone and it depends on how it's connected.
The current sound is thin and lacks imaging and depth. I have Grado SR225 headphones. I think the fix I'm looking for would be a drop in replacement for the current op-amp.

The alternative is a dedicated headphone amp.

Thanks for answering.