I suspect the 501's tested to be the "thin black ringed" ones, known for weaker bass. The most evident differencies on graphs are bass response and some (probably random) irregularities on highs - I mean the dip (if we consider 501 and Orpheus to be the reference) around 4K on 701 and 601. So... why pay more?
I must point to importance of ear coupling for achieving the deepest bass response and good soundstaging - so, shave your head, put well sealed leather cushions on your 501 (i don't care how would you get some), apply some grease and enjoy the perfect bass along with the most beloved by many folks in there highs and mids! No kidding (or may be just a bit) - such morbid deeds will cancel out much of that uncute bass rolloff.
BTW, congratulate me - this post gives me the 100+ status