As mentioned, the noise of the fans on the DD670 was pretty high. The five fan modules seem pretty proprietary, and probably a pain in the ass to swap them out for something quiet (and expensive, relatively). So I just removed all but one fan module in the middle and temperatures are fairly stable, with less noise. I can still hear it at night when there isn't anything else making noise, but not with normal volume levels from the television and 2 and 4 year olds running around.
I also removed the battery unit and related (nvram?) card. I'd imagine it has more of a use with DD OS, but not how I'm using it with Debian, mergerfs, and snapraid. That's what I'm testing, at least. Next step is to move the existing md RAID6 array from my current fileserver to this one, get another drive or two, and migrate the data. I'll probably use a friend's fileserver with actual free space to help that along so I can fully switch away from mdadm. The SAS card only supports 2TB drives, and I'm thinking that the backplane does as well. The card would have been an easy upgrade, not so much the backplane. Even so, I'll still be able to add another 8TB to my current usable 12TB. By the time that fills up, I can look at other options, like a Lenovo SA120 disk array.
I have a Xeon L5630 on the way to test in the system to replace the E5540. Whether it works or not, I'll switch fileservers after that. And then I can repurpose my original fileserver to be my firewall. Also on the way are a couple of different drive caddies to test - one R510 type and one C6100 type. I could use at least 4 for the top row of drives for now, the bottom two rows I can insert easily enough without a caddy.
Edit: About an hour after posting this, the CPU arrived in the mail. The server doesn't like the L5630 though, just beeps at me when trying to boot. I couldn't really find any information on what CPUs it supports. Not a huge deal, just would have been nicer to have a lower TDP CPU.