What's the technical explanation for these sorts of differences? Is this impression based on level matched listening, or does the dongle just not drive these very loud and so the bass seems week due to equal loudness contours and the dynamic range poor because there's less gap between your listening volume and ambient noise floor?
Working from first principles, to reproduce bass frequencies, the diaphragm has to oscillate slowly which implies the magnetic field strength reproduced by the voice coil has to be sustained for a longer period of time.
Magnetic field strength is proportional to current flowing through the voice coil. And the Sennheiser HD650 has an impedance of ~ 500 ohms near its primary resonance. Since I = V/R(1.03/500) is very low when compared with driving low impedance headphones, there is not sufficient juice(current)?
P.S. My explanation is very crude. A graduate/PhD in EE might be able to explain the theory behind it.