First off congrats on a beautiful β22 Jack! That one is really stunning!
My β22 is set at a gain of 7 from what luvdunhill says here, but I thought it was 5 from what he said while he was working on it. It drives my K340 effortlessly, it also drives my Chocolate Donuts effortlessly, and they're even more power hungry than the K340.
I would think that a gain of 2 would drive the Senns. without breaking a sweat. I have to turn my volume down a fair bit when driving my HD 580.
That said, my source has a high gain.
I find that the amp drives low impedance phones just as well. My deep woody CD3000s sound amazing with this amp.
Again congrats on a very, very nice amp Jack. Welcome to β22 heaven.