Not sure about those particular models but I have had the A-S500 (which I gave to a friend) and currently have the A-S2000 and it is superb. I don't use it for my headphone amp so I can't comment on it but for driving speakers and the build quality I can't say enough. I also have the CD-S1000 (bought used) and it is more musical than many of my dacs. I feel like I need to unbox my cds that are in storage and listen to them again instead of wav/flac files played through several of my dacs.
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From what I can see, the A-S2100 has (4) 22000 microfarads power caps and a fully balanced design while the A-S1100 has (4) 18000 microfarads power caps and is unbalanced. Not much different between the two from what I can tell.