Burson Audio V6 Vivid Dual (can't close unit), LME49720HA Metal Can (can't close unit), LME49860NA, OPA1612, and OPA2111KP are all good upgrades. Do not buy any of these on EBAY from China sellers. They will all be fakes. The OPA1642 is an ideal choice if you are looking for a neutral opamp with a hint of warmth to the mids. It pairs perfectly with the NX3S. If you want a neutral opamp with more dynamics, and a more punchy bass response, the OPA2111KP is an excellent option. The Burson Audio V6 Vivid Dual makes this unit sound like a 250 dollar headphone amp, but you can't close the unit with it installed. The OPA1612 or OPA2111KP would be the best choice for standard sized opamps.