This was not my experience. I think the x2 murdered the AKG 7xx and 712 in terms of quantity of bass, and to get the AKGs to volume match the x2 for bass, the treble bump made me want to pluck my eyes out from their sockets. I love the K712s dearly, and owned them for about 4 years, but bass was not their strong suit. Mids were solid, stage was big, imaging was decent but inaccurate (from my game playing experience). The x2 had a pretty big stage too, but was more heavy than was comfortable for me (akgs easily rank among the most comfy headphones I've ever worn), and I felt the AKGs were more detailed.
One big problem, the AKGs were kind of ineffecient, especially compared to the x2.
I dearly love the Hifiman 400 series, and have the 400i, 400s, and 4xx. I think the 400i scales best of the three with better amps and dacs, but the 400s and 4xx are more efficient and sound better with entry level or lower power units. I think the 400s sound is a bit more laid-back and pleasant, generally, but I think the warmth bloats the bass a tiny smidge. The 4xx has solid bass, but to my ears was a bit brighter (reminds me of an entry level 560, with a bit more bass), and the mids felt a bit hollowed out compared to the 400i and 400s. I generally recommend the 400i unless you know whether you prefer warm or bright to neutral, or if you want a big stage, which isn't a weakness of the line, but it's no strength either.
I actually would also forward the Hifiman's, but would also throw out the hd6xx, but amping might be a problem for that. Another planar option in the price bracket is the Fostex T20/40/50rp series. I rather enjoy mine, but again, need an amp. Though, for the price of the t50 or he4xx, you could probably get an amp with them and still be under the 250 mark.
