I own the HFI-580, 680 and 780 (2 pairs). The 580s have more bass than the 680s but less than the 780s and are not as detailed as the 680s or 780s. Still a fun can. The most balanced sound among the three is the 680.
I also own the HFI-700 and it too is balanced in the spectrum, with more bass than the 680 but less than the 780.
IMO, all Ultrasones have bass, that is not their weakness. The 780s also have very piercing highs that make your eyes water, out of the box, along with great deeeeeep bass. The highs and lows seemed detached from each other while still being prominent. Burn-in and a re-cable took care of that.