I have looking around the the Head-Fi forums for a couple weeks now and I thought I would chime in a bit on the AC20s as I have recently had a few emails back and forth with Westone Customer Service. A few weeks ago my church offered to purchase AC2s for members in our team as a way of saying thanks to the guys who play in the band every week. We have been using inexpensive universals from phillips, with myself using fairly inexpensive UEs universals. I think some of us were using iPod head phones... so clearly they were not audiophiles.
I was pretty excited about the possibility of CIEMs; as the cost has always been a deterrent. Some are more then my entire guitar rig in cost. Last week made the appointment with my audiologist and I was checking Westone's website. They had replaced the AC2s with the AC20s, I am assuming due to CES. So I sent an email to see the differences and which one I would end up getting. Essentially, the AC20s are the same as the AC2s. The core differences are the color options, which we not available before, and the new cables that are like the Shure/UE cables. As a point of fact, I was informed by Westone today that the dual bore reference to the AC20 in the press release was indeed a typo. So that's the jist. Just a upgrade for the cables and the colored acrylic. No dual bores, no re-tuning. There is a significant improvement to the ES series over the AC series. They are designed for budget CIEM for musicians, the AC10 are for vocalist.
I will give a review when I receive my pair. Some time next week I guess.