I would love one of these beasts in my apartment. Your price is also fair for the hashrate and PSU built in.
Anyway to undervolt/underclock these things so they are as quiet as an Antminer S3?
You can certainly underclock and undervolt them, and you can turn down the fans, but I've never heard one in person so couldn't comment on whether it would be as quiet as a S3.
I sorta maybe doubt it, the fans on these are pretty small and loud.
I looked at some photos of the settings and there is a way to limit the power from each PSU at 500W, so wonder if running it at 1000W instead of 2600W would make it quieter. Too bad you don't have these with you physically, so you could of checked.
But anyways good luck with sale.