You should try going for a spin in a standard 540i, super smooth and refined. You can barely hear the V8 exhaust note at all - personally I find it too quiet. Thankfully the 840 is a tad more raw and the exhausts free up the sound a little more (mine has standard exhausts).
I can't really compare with my Omega as it has a crappy aftermarket exhaust that sounds terrible, but I do find the V6 getting a bit harsh at the higher end of the rev range. The inline-6 fitted to my dad's 330 certainly feels loads smoother right to the red line.
I must admit, I drove a 3.0 Omega years ago that I quite liked the sound of, and my mate had a 2.5 V6 Vectra years before that, and it sounded nice (100% standard). However this one doesn't seem to have it, I'm not sure if it's because it actually sounds different, or because my interpretation is different. I hadn't owned a V8 at the time I heard the previous two, so maybe that's had an effect.