I too love the K501's. I only had a pair of SR60's when I came to Head-Fi. My first purchase (full-sized) were the 501's. After reading all the posts on the K501, I expected the bass to be anemic. It isn't. In fact, it is only somewhat lacking when I listen to music with electrified bass, and then not always. The quality is there, it's just not visceral. Since then, I have searched for the complementary phone for the bass heavy music. Currently, the contenders are the DT880-03 and HD580 (would have gotten the re-badged Jubilees (HD600), but didn't like the bird droppings look). On occasional recordings, the cymbals bother me on the 880's. The HD580 seems a better overall phone frequencywise, but I'm finding that the added bass doesn't outweigh the 'missing link' that the K501's have. The DT880's have more of this missing link.
I don't know if it's what everyone is calling soundstage or what. (To my mind soundstage is more akin to separation). What I find that the 501's do so well is render what I experience as 3D, or depth, or ambience. I find the strong point of the Senns is their timbre. They register instruments very realistically. But after switching to the 501's from the 580's, I notice that the 'sound stage' has collapsed into 2D compared to the K501's. The more I listen, the more I find that to my taste, this 3Dness or depthience that I like, and that the K501's do so well, is hard to replace. I keep going back to the AKGs. I realize that my equipment chain isn't the equal of some of yours, and I have yet to hear more top of the line headphones, but at my stage in the quest, the K501's please my tastes very well.