The thing with more expensive headphones like the HD25 is that it will be more revealing of your music and sources and some more expensive headphones need some serious amplification that a DAP will not offer. The costlier the headphone the more it is likely you have to improve you listening set up. If you want to upgrade be warned for your wallet... 
 
As for the PX200 II, it is good but my Pioneers and K240 DF leave it in the dust when they are well amplified. If anything the PX200 II is a very good sounding headphone and on its own better than it should be if you just want to enjoy your music. I am still impressed by this little Sennheiser...