Check out the closed headphone options in the Summer 2014 Buying Guide (linked on the right side and bottom of this page), there are a lot there that you may not have heard of. And before you ask, I haven't heard most of them.
The Alpha pot should be the obvious choice for anyone who uses low-impedance, sensitive headphones. That's what I had and it was perfect for the Audio-Technica AD2000 and Fostex TH900, I was able to achieve any precise volume level I wanted. Also not to continue to beat the horse on stacking, but doesn't anyone read the manual anymore? It's stated pretty clearly in there that it shouldn't be done.
The HD598 and HD800 sound more similar to each other than either of them do with the HD650 (the HD598 and HD800 both have angled drivers). I haven't heard the NAD HP50. The HD650 are more close-up and full-sounding headphones than either the HD598 or HD800, as are the KEF M500 (but the KEF are slippery and could be prone to falling off).
Where do you live btw? I'd guess you're in the USA and there are a surprising number of meets/shows coming up around the country very...
^ I pretty much agree with your assessment of the LCD-X as well. Got a chance to hear it through HeadAmp's Demo Program and just couldn't bring myself to think it was all that great. Really curious about the HE-560 now. I'd almost written off HiFiMan's headphones based on previous experience with the HE-400 (and only briefly the HE-500) but the HE-560 seems intriguing based on your review.
Westone has generously offered to host a meet for us Head-Fiers this summer at their company headquarters in Colorado Springs, and to get things rolling they'd like a number on the possible turnout. So please post here which date in August you'd prefer between 8/16 or 8/23, along with any gear that you'd be bringing. No need to bring equipment, but as always it's highly encouraged, because the meet would be pretty boring without much member gear.
For the meet newbies:...