After agonizing over whether to drop the money on the mods for this chassis after seeing your success, I decided to try one last quieting option - an inline resistor to bring down the speed of the PSU fans. So I yanked one of the PSUs, disassembled it, unplugged the fans, and wired them up to an old AT PSU for easier testing purposes. Unfortunately it appears that they just won't operate with less than 12V, and they are 38mm fans, which is not an easy size to locate cheap, quiet replacements for. Guess I'll have to take the plunge on modifying the case when I redo my server rack.
I'm definitely looking forward to seeing what other folks here do with these servers.