Pass xa600.5, Marantz 9S2 or Mcintosh MC501/MC601?

Pass Labs. A very transparent amp.

I've compared an XA-30.5 to a McIntosh MC402 on Thiel 2.4s and the Pass beat it in every category, except bass control. I didn't get a chance to do a head to head, but I've also tried out an X250.5 and it was close enough to the XA-30.5 that I'm sure it would also handily best the McIntosh (while also addressing the bass issue). To be honest, I was shocked at the extent to which Pass beat the McIntosh.

Another vote for he Pass Labs.

I have Tyler Acoustics Linbrook Signature system speakers with a Pass XA30.5. They are definitely a synergistic match for each other. At higher volumes (90db) the needle moves past noon, so at some point I'd like to move up the line and get a more powerful Pass amp.

I can't comment on the other amps because I've never heard them.

I owned the Marantz and found it to be too polite. I own Pass now and have no desire to upgrade my amp anymore, having been through 5 amps, some a lot more expensive than the Pass. One was a McIntosh, which has a warm mid-range but somewhat flabby bass.

IMO..having owned all three makers..Pass Labs without a doubt