Do you know what sort of levels you listen at? I only ever listen to my HD6XX out of my phone which has a pathetic output of 0.3V, "almost useless" according to amir, so I know I listen very quietly and I've tested massive boosts like +12dB at 20Hz and never heard any distortion.
I don't listen to much music with bass that low though so maybe I haven't experienced what it is supposed to sound like.
I don't listen to bass for the sake of strong bass, but I like to EQ for acoustic sources (classical, folk, etc) so that there is clean bass in reserve to create a full FR and convincing fidelity. (I listen to electronica, too, but bass is almost like a separate "instrument", not just a frequency range, with that.) The point being: dynamic range down to the sub-bass, separation, accuracy vs. distortion, realistic balance with mid's and high's.
On that score, the DCA planars were a revelation for me. Plenty of clean, undistorted bass, easy to EQ, wonderful separation. I also have a Sundara. It needs a little bass boost and accepts EQ gracefully. I hear a little more resonance in the mid-bass, but find it pleasant with bass guitar. If those sound like your values, then give the planars a try; or perhaps there are dynamic drivers that do the same thing. Whatever the reasons, I find it hard to go back to the Sennheiser HD650 bass b/c it sounds congested and underpowered to me now.