I'm curious as to how this plays out, pun semi-intended! I have currently shelved plans to buy the E453 (which, of course, is essentially a 61 key version of the EW400), but one reason why I would is the direct USB audio out -- but if it doesn't work well, then it isn't worth it.
I know my E433 definitely has some absurdly low preset volume settings on some of the sounds, but it is mainly sounds in the XG-Lite range of voices. When I was looking at the E453, it seemed they corrected this issue, somewhat.
As for adjusting the master volume knob, danmc said that he turned it all the way down to make sure he was using the digital output, and he said he could then hear nothing out of the speakers, but was still recording music through the USB -- Here's the quote:
"For the record, I'm sure that the recording path is via the digital audio because I've tried turning down the volume knob on the yamaha so I can't hear anything at all and I can still see stuff on the VU meter and can record. That was my sanity check to make sure I wasn't doing something silly like yamaha speaker -> laptop built in mic."
So this makes me wonder, how do you adjust the overall level output through the USB audio? Yes, the main, dual, and split voice levels should affect the volume of those parts individually, and I would imagine that the Master EQ should work, as well. But you'd think there should be some sort of master output level for the USB. Maybe it's in one of the function settings?