My Pico Slim just arrived today, after a short trip to the Post Office to pay the taxes. A few initial, subjective impressions, out of my iPhone 3GS/Oyaide LOD (thanks Currawong), driving SE530s:
Volume Control:
The volume control is excellent. The volume pot goes from a 7h30 position to a 16h30 position. I can just barely hear music at the 8h30 position, and the lowest I'd ever actually see myself listening is at the 9h00 position. The loudest I would go to is about 14h00 on the Slim dial, anything else is really too loud for my tastes. I find it very easy to find the level I want, and I do not hear any "clicking" when turning the dial, except possibly at the very lowest end of the dial, below 8h30 where I can't hear the tunes.
For those who have the iPhone and know that the lowest volume setting there is not really that low, I'll give some perspective by saying that's at about 12h00 on the Pico Slim dial.
A side note: driving PortaPros
I can first hear music at 10h00, consider 12h00 to be a "low" volume, and near 16h00 a "high" volume.
Turn-on/turn-off noises:
Yes, I can also confirm the "thump" when turning on the Slim with IEMs plugged in. I'd describe it as a muted pop. Turning the Slim off gives a very short sound that starts high in frequency then goes down. I'm no engineer, but I want to say it's the sound of a capacitor discharging (those in the know, please feel free to correct me on that). Plugging/unplugging my IEMs into an already powered up Slim gives no pop or thump that I can hear.
Hiss:
My experience tells me the SE530s are very sensitive to hiss, moreso than the TF10's I used to own. I can hear a very faint hiss at the 13h00 position on the Slim, which is roughly my normal listening volume. Turning the pot up to 14h00 (the upper limit of what I'll listen to) gives a hiss roughly equal to my 3GS. 15h00 or higher seems to give a rather high amount of hiss, but I don't care about that since music at that level would deafen me. All of these observations are identical if the Slim is connected to the iPhone on pause, or with nothing plugged into its input.
To compare to the Pico, I distinctly recall NOT hearing any hiss on the low gain setting at either low or high listening volumes.
EDIT (Apr 23, 01h28): To clarify, the above observations are when no music is playing, and I am actively listening for hiss. After having listened to the Slim with music playing for a few more hours, I feel ready to add the following: At lower volumes, there's NO hiss. At medium volumes, I don't notice any hiss during music playback. At high volumes, the recorded hiss in most of my music (which no amp is going to get rid of) is much louder than the hiss inherent in the Slim, so I think it's fair to say that I don't notice any added hiss during music playback. (Low, medium, and high are my subjective definitions, of course).
A side note: driving PortaPros
No hiss here!
Preliminary Musical Thoughts:
I can only make the broadest of comments based on an hour of listening, so I'm only going to share the first thing that hit me with the Slim: the bass is greatly improved over the iPhone's HP out. It's better defined (i.e. not boomy) and has more slam. There are other sonic differences that I'm still trying to put into words, but I am reminded of the fullsize Pico in that I don't hear any coloration and the music just sounds "damn good".
PS: I did try the Parasound 1500 as the DAC, which I think makes for the largest DAC-to-amp size ratio evar...