Well having only heard the pico slim and the protector ill do what i can brad. I owned both of those amps, was pretty high on both preorder lists and got them both in the first shipments that went out. I wanted to be able to compare them both myself. though to be honest, from the comments i was reading from several long standing members, i though that even though it was cheaper, that the slim was going to be better.
but my experiences were definitely different. I got the protector a good month ahead of the slim and got pretty used to its sound signature. So when the slim arrived and i finished charging it, i notices all the changes right away. The first thing you notice is the bass. On the protector is goes quite a bit deeper, at least with more authority down there, and the bass has more impact, i think because it can go deeper. Bass drums sound quite a bit fuller because of this, and bass samples in like lady gaga sound much fuller as well. Also the slim, has a slight bloat to the mid/upper bass, though i probably wouldnt have noticed unless i compared it to the protector.
The next thing i noticed was that on the slim everything sounds a bit more congested than on the protector. not really a smaller soundstage though. I have a hard time describing what i heard, it wasnt really a difference in instrument separation either, just sounded almost like on the protector there was air between all the instruments but on the slim there was something thicker than air between them all.
In spite of all that i made myself listen to the slim for 2 weeks, and had the charge it several times in there. I was hoping that burn in would change it a little but, but it didnt, whenever i listened to it, i would think about parts of the music i was missing from the protector.
But obviously all the is IMHO
the protector is also extremely transparent, and source upgrades make huge differences. Between the iPod lod, and using my dac19 from audio gd.
i wish i could provide comparisons to more of the other amps, but alas i havent heard them.