A thing to bear in mind is that open series and closed series Audio Technicas having the same number are two totally different beasts. I prefer the AD900 to the A900LTD for the majority of genres of music. It is well known that that open headphones make the music sound more natural and the mids on these cans really shine. But like I said before, the bass may be lacking for some (one of the only reasons I switched to the A900LTD)
But everything is subjective really, to me the AD900 was in a better league than the K701. To me the K701 were nice and pleasant, albeit a bit bland. Perhaps amping, or lack thereof of the 'correct' amp possibly had something to do with it. In contrast, the AD900 leaned towards the more fun energetic side of things.
I was using a headsave classic with the K701. If you have to get the K340, K701, HD650; I think you would to spend at least $200+ for the 'correct' amp to get the true benefit/magic out of these cans.
I'm not belittling any of the big boys, just that depending on your situation/budget you may do well with one of the AD series. Maybe the DT990 as well, I've heard a lot of good things about those phones; would like to try them out someday.
Musician's friend has the AD900 for #239.99. I believe they have a 45 day return policy, so there is one option for you. Try em out and see if it's your cup of tea. Although I bought my AD900 through pricejapan.com for $227.00 shipped, but they do not have a return policy I believe.
But as always, audition first, that is your best review