Just to close this out, I now have 16-core w/ ProVega II and sound level is identical to the 24-core. Maybe I just have really good ears (???), but, to me, the 7.1 MacPro is not silent. No complaints. Love the system! Just sayin', that's all.
There's actually another thread on this topic that I didn't discover until after I started this post:
We call it "selective hearing" in our house.
forums.macrumors.com
One of the posters uses an iPhone app to measure sound level. I downloaded the same app, and my system measured 25db (+2 over ambient late at night in a 'quiet' room), which is right around where Apple says it should be (I came across reference while researching, but didn't save link). Fully recognize iPhone app is not calibrated to reference, but the delta (+2) can be more trusted.