Before you trash the system, I suggest you try a set of 'phones with higher impedance. The 650's, some models of Grado, and the top AKGs all have very low input impedance. MAYBE if the output impedance of your headphone jack is a bit high, that would explain the lack of bass.
ONE other hypothesis: try going back to the 650's original cable. If your current cable's wires were reversed, putting one side out of phase with the other, you'd get EXACTLY the results you're describing.
ONE other hypothesis: try going back to the 650's original cable. If your current cable's wires were reversed, putting one side out of phase with the other, you'd get EXACTLY the results you're describing.