Lynn
It's not only about the sound, but it also is supposed to aid the processing of your 64 bit plug ins. 

I'm afraid you making the common mistake of conflating 64-bit data width with 64-bit signal amplitude precision. 64-bit data width aids processing speed and efficiency, but has no effect on sound quality. The 64 bit double precision engine allows for more precision in the summing and processing of audio samples.

The 64 bit option just affects the depth of precision that the audio engine functions at. It does not change the fact the you have a64 bit program and plugs that can access more ram. 
 
I can't hear the difference either and I have had it off for the last several months. however I still use it for export just in case some magic happens. LOL

Sorry, ignore me

Hello -Since all 60cycle hum reports and *certain fixes* seem to be related to *upgrades* I am chagrined to report that I am experiencing the hum with a BRAND NEW INSTALL, not an upgrade. I've used Sonar since V3 and am upgrading from 8, and have built three previous DAWs. Other than crashes at the most inopportune time I have never had an issue such as this. I hope I'm not missing something elementary. 
 
I've been tracking the forum recently and noticed this thread, because I've been preparing for an upgrade to a new super-fabulous  StudioCat DAW (Pro Tower) and X3. (What a machine - thanks, Jim!) So last night I installed my Profire 610 interface, then X3, X3b, and then X3c. Fired up Sonar, dinked around trying to make sense of this new environment, loaded a project and immediately got the hum. Came to the forum for further research and first read about the VST scan for true upgraders and figured that didn't apply to a brand new install. Then I read about the 64 bit precision engine button *fix*. Remembering that I had engaged the button while dinking around after the install, I then disengaged it after reading the thread. Closed Sonar and reopened the project - still hum. I rebooted, fired up X3, and upon loading my project was immediately greeted with the hum again. And, let me be clear - the hum only appears after loading a project, and ends when closing a project, so I don't *think* there is any interference from another device.
 
BRAND NEW INSTALL. Any thoughts, my brothers? I appreciate any feedback - this is actually kind of nuts... TIA!
Dono
Frisco, Baja Oklahoma