Harry - I'm curious, did you investigate the V&H Monster Rounds compared to the Monster Ovals to learn which is louder? I went with the Ovals because at cruising speeds they're supposed to quiet down as well.
Kirk,
I watched as many videos as I could find, and had good speakers hooked up to the Mac to try to get a feel for the sound output. I know, I know, it isn't the same as being there in person, but you take what you can get sometimes. And where we live, the odds of finding bikes with those pipes are staggeringly low!!!

I also did a lot of forum reading and perused other websites that talked about them. I had the older ovals on my '07 EG and we loved them, but they were loud enough to make conversation tough. There are also comparison reports or reviews out there if you'll look hard enough, all of which I know you know already.
Finally I called V&H and talked with them. They aren't the best at customer service, but the sales guy backed up what the research had been telling me.
Ultimately it was all of the above plus V&H's own literature that convinced me. Here's a quote from their website:
Like it’s cousin the Monster Oval, the new Monster Round produces that deep and mellow rumble that’s winning the hearts and minds of touring riders who put in the long distance miles. Perfect for enjoying your stereo on the highway, the Monster Round features blue proof technology and a CNC machined billet end-cap for an impeccable appearance.This is my 4th set of V&H pipes and I tried to feel as right about the decision as I could before I pulled the trigger. The Rounds don't come cheap, but in the end, I made the right decision. We can talk without yelling and the stereo is too loud at 2/3 volume.