I've used the iwaki equivilent on a closed loop; I think it's MD40rxt. I spent way more on the iwaki because on that particular pump model the japanese iwaki is supposedly quite a bit quieter, something to do with the hydraulics of that casing and impeller. BTW, when I was on the phone with iwaki, they guy told me that the circulation pumps (non pressure) really do run better with some head pressure, so you might try putting a gate valve on it and just putting enough pressure to quiet it down; hopefully not losing too much flow.
Saltwater might actually make a difference, at least my new pan world 50 pxx return pump (different system) is quieter with salt water; not much, but some.
The only pump sort of in that flow class that I know of that's much quieter is the CSL velocity T4, now owned by blueline (championlighting) I used a smaller one on my skimmer a while back, and it was amazingly close to silent. Man does it run hot, though, so if heat's an issue forget it.