I suspect the Shures would assume your K550's position, but I've not heard the AKGs so can't make that prediction with confidence. Funnily enough, originally I went to buy the K550s but the 1540s were in stock so I skipped them entirely and ended up coming away with the Shures! Kinda wish I'd at least auditioned the AKGs now to have an impression of how they sound. Alas.
The HD650s should be on my shortlist of open cans, too. They're so universally praised, and so (relatively) inexpensive. I've heard others liken the 1540s to a closed HD650, but again, as I've not heard the 650s, so can't verify!
As for the Y50s, I think they're fantastic for their price. The only issue I have with them is that comfort is, well, typical of on-ears i.e. the outer parts of my ears start to feel like they've had things pressing on them. Have you thought about the M50Xs for work? They're not much of an upgrade on comfort over the Y50s with the stock pads, but stick some MSR7 pads on them and it's a much plusher experience.
