This instruction and samples shown here is for the DH2730, the general idea is also applicable to the newer DH37x0.
When powering off a DotHill you can log into the WebUI and use the following steps:
1. Login (default username / password = manage / !manage)
2. Click the "Manage" button
3. Clink the "RESTART SYSTEM" link
4. Choose "Shut Down Both RAID Controllers" from the drop down box
5. Click the "Shut Down" button
6. You will then need to physically power the unit off.