I have the X600s, and I like them. I find them powerful and neutral, with good extension at both frequency extremes. Notably, the highs are nice and detailed -- feathery. As Class A circuits, they consume quite a bit of wattage and run pretty hot. While their Class A architecture suppports a very smooth midrange with no trace of solid state graininess, they may not have the ultimate liquidity of subsequent Pass models in the XA and X.5 series (nor the Aleph series). Sorry, I have no experience with the Mac 501s.
