The GUSTARD H10 headphone amp is very popular among Head-Fiers. At $350 USD it delivers great performance and it is also well loved by DIYers.
Among the many DIY tweaks available for the H10 op-amp upgrade is the easiest and most effective.
Here is one of our customers who upgraded his H10 with one pair of dual and one pair of single SS Opamps:
Here is the active and ongoing discussion about the GUSTARD H10 on Head-Fi: http://www.head-fi.org/t/735828/gustard-h10-high-current-discrete-class-a-output-stage-headphone-amplifier/2775#post_11775092
Happy listening and modding!