What has been said about the Mad Dog's bass and why it can be perceived as light is true. Compared to many other headphones, it does have fairly clean, tight bass. The harmonic distortion isn't the lowest ever, but can be noticeably better than some other headphones known to have more bass than the MD (harmonic distortion can give the illusion of there being more bass than there actually is). Then there's the fact that the bass is tuned to be fairly neutral, so if you're used to a bass-heavier sound, it will sound light. On top of that, you have the fairly small ortho driver in the T50RP, which isn't going to put out much in the way of physical impact even if the FR is neutral (less air movement).
And while the Modi/Magni is a good entry setup, you will probably get a "bigger" sound with better, more powerful gear. The T50RP scales pretty well with equipment and is surprisingly hard to drive to its fullest potential.