I have two integrateds, one a Peachtree Grand X-1 (class D) and an Audio Research VSi55 running KT120 tubes. Through first Proac D2, then Wilson Benesch Arcs, now BMC PureVOX speakers, I can't tell the two apart other than that the Peachtree is more powerful and will play louder. At volume-matched levels, I cannot distinguish between the two. The only time they sound different is if I engage the tube buffer (running two upgraded Cifte 12AU7) on the Peachtree, then it's actually a little warmer than the Audio Research. But with the tube buffer off, it's apples vs apples. It's my opinion that the ARC VSi55 puts out more than acceptable sound quality, ergo Class D has officially arrived, at least in my house.