I suspect the inconsistencies in tone and pitch you hear through your 7506's isn't so much to do with the behringers per se, more that you are actually hearing faults in your mix through a pair of 'accurate' headphones. The same mix through a decent set of monitors would reveal the same faults, which is of course exactly what you want.
I would say, that unless you write dance music, or have a lot of airspace to fill, an 8" bass cone isn't neccessarily the best choice, as in a smaller room that amount of bass can cause problems. True, larger speakers usually go lower in frequency response, but there really isn't much in it in an average mix, and they tend to be much more expensive in a given range form a given manufacturer.
Have a look at the manufacturers I mentioned above as well as the mackies. Personally, I just went through the same dilemma as I had a pair of M Audio BX8's but knew I was missing stuff, as my mixes didn't translate, even with acoustic treatment. In the end I plumped for a pair of tabletop Genelec 8020's. A year ago I never would have considered buying a pair of monitors with 4" bass drivers, but they are quite astounding really, sort of like the first time I heard music in stereo back in the 70's. Now I'm happy as a lamb, knowing that what I hear is what I'm mixing.
Do take a range of familiar material with you to a shop and try and get them to let you do a listening test after shop hours when it's quieter if you can.
Oh, and welcome to the forum!