Never heard classe but I have owned the others. I think pass is the best of this bunch personally. I did not care for the ayre v5xe it definitely is not dark sounding in my systems. I do love the k1x preamp though. Sim audio is also very sparkling on top if not a little bright and maybe just a little light in the bass to my ear but still really good, this is based on the sound of the i5.3 integrated. I think your choices are great just wait for a good deal to come along on any of them and I would bet you will be happy. Though I personally did not care for the ayre amp much.
"Neutral/Clear Amp vs. Dark Amp"
It seems that of the many reviews I read about amps within my price range (under $5,000 new) they always end with some reference to a "dark" sound. That is exactly what I am NOT looking for. I'm looking for a clear, neutral sounding amp that won't add anything other that a clear representation of the music. I'm thinking maybe Classe, Ayre, Pass, Sim Audio? I'd like to hear other recommendations. I'd also like to stick with SS, as I have a deHavilland preamp and it mates well with SS amps. Speakers are Von Schweikert 4.5's and I use silver wire throughout. Listen to digital and analog, all types of music. That's the key. It has to sound like music, otherwise why bother? Currently using a McCormack DNA-1. It's a good amp, just looking for a significant upgrade. Another choice is to send the amp back to McCormack and have them do some significant upgrades.