If you are mixing with only headphones and don't mid buying 2 pairs, then I think using a pair of 240 type and a pair of M50 would work best. I use both AKG K240 (600 ohm 1980's vintage) and ATHM50 to check/adjust my mixes (I don't mix with headphones). They sound very different so this works well for hearing problem areas of a mix. For me if the mix sounds smooth and defined with a big balanced bottom end on the M50's and Ns10 sounding on the K240's then my mix usually plays well on all systems. You can get the bottom end right on the 240's but you have to learn them as the bass is not pronounced, just as you would have to approach mixing differently if using only the M50's. The low end of the M50 somewhat ( and I emphasize somewhat) simulates what you would hear on a full range soffit monitor system in a high end professionally designed control room. I did recording and mixing work in a Tom Hidley designed "Westlake Sound" recording studio with soffit mounted Westlakes with 18 inch Gauss woofers. They moved alot of air, really THUMPED (long excursion). The extreme bottom end and the transient attack of low end notes was SCARY. I had never heard the real bottom end spectrum like that until I worked in that room. The M50 kinda reminds me of that sound (without the transient attack) although not nearly as defined, after all they are only $100 headphones. But the M50's give you an idea of what's going on in the bottom end.
