I recently bought a '501 from Red Rose Music in NYC. The salesman there said he's used both and the differences between the two are very slight. A bit more refinement in the 901, but nothing major. So, the comments you find on the 501 should be useful to you as a guide. I am pretty amazed at the 501. Even not broken-in it sounds really open, smooth, detailed and clean. The Glider it replaced - which I thought sounded good, and costs about the same - sounded boomy in the bass and too smooth by comparison.
