I recently tried pairing my newly bought (secondhand) HeadAmp Pico Slim with my DX90 and these are my impressions:
The DX90's onboard amp has surely more power when driving full sized headphones. So the sound comparisons are while using my FAD Heaven VI. The soundstage has equal depth maybe the DX90 winning by a little due to a more laidback midrange. The DX90 wins in soundstage width noticeably but most apparent was in soundstage height even with IEMs (really wasn't expecting that). The DX90 has slightly more midbass and more subbass than the Pico. The Pico Slim wins in clarity and articulation. Although less spread out, instruments are better defined and the DX90 seems muffled in comparison (the DX90 is not muffled at all). Bass had more detail on the Pico and was tighter but was quite lean. This would probably pair well with IEMs with strong bass as I hear better control from the Pico. Again, the Pico Slim won in clarity of the mids but pushed it towards the sweet but slightly cold side as opposed to the less cold mids of the DX90's onboard amp.
Overall, if you already have a DX90, there is no need for a Pico Slim unless bass is too much with mids that are muffled. If that's so even with the DX90, you should probably just change your headphones…
It gets more interesting when it becomes the iPod Video 5.5g with Pico Slim versus the DX90. The DX90 wins in overall technicalities but I just love the tonality of my Heaven VI with the iPod and Pico. Less bass and soundstage but better clarity while still keeping warm and smooth mids. I probably wouldn't have bought the DX90 if I got the Pico Slim first. That being said, I enjoy both setups equally and will continue to use both depending on my mood.