[emrr]

You can always go ghetto like me and make a passive transformer interface box to tack on the output of the P3500S when you want that sort of vibe. I have UTC LS-30's and Western Electric 111-C's set up this way, so I can use them when and where I like. They definitely wouldn't fit in any 500 series rack!

P3S or P3500S, I'd not loose sleep over having one versus the other. Buy the 500 version if you are entrenched in that format, likewise buy the rack mount if it makes more practical sense for you. I have 3 different variations of the FCS compressor, and they all deliver the same performance, the newest versions expanding the bells and whistles fabulously.