In order of dissapointment
- Sony EX-71. I know they're cheap (~$50), I know theyre a mass market brand, but boy are these bad. They're uncomfortable (although if you can live with other canalphones, maybe you won't care about this) they're highly microphonic (pick up sounds from cable) and you can't eat anything while you listen. Worst faults are with the sound signature though, which is just bad muddy bass, shrill highs, garbled mids.
- Sennheiser PX200. Bad bad bad. Still cheap, but I can't believe that sennheiser sells these. One of the few phones in this price range that sounds worse than some of the better pack-ins (not the very worst pack-ins.)
- The $50 "upgraded" headphones apple sells on their site for the iPod, the pseudo-canalphones. I didn't buy these, I got them for free since Im a known headphone nut in my friends circle and a friend told me that these sucked so much he'd give them away, this from a guy who is not an audiophile at all and thinks Klipsch speakers are high end. So I had no / low expectations, but WOW, these are worse than the EX-71, worse than a $20 pair of MX500 buds, hell, I can't think of a worse name brand pair of headphones at any cost.
I know some people will crucify me for this, but after hearing comparisons to the Grado SR-60s, I tried a pair of KSC-35s a couple of years ago, and while they aren't bad, they're outclassed by any Grados. Doesn't count as a big dissapointment, but still, theres a lot of improvement to be had if you can afford to get closer to the $100 mark.
Also, I will straight out say I'm a Sennheiser apologist (no that doesn't mean I apologize for liking Sennheisers, it means that I would defend / recommend them.) and I think a lot of people who bash the HD6x00 line are people who would also not like many high end stand speakers, such as mounted monitors like the Dynaudio Contour 1.3 SE or Special 25s or Talon Hawk's or the like. I think the Senns share the dark / warm but tonally accurate signature of high end Dynaudio stuff, they need to be properly powered, they need to be fed from a high end source, but in the end they are tonally truthful, transparent and accurate. They're not harshly analytical, nor do they gloss over details, and they don't add artificial excitement to music. I like Grados, I like AKG 501s too, but in the end, I can listen to and enjoy music the most with Senns because theyre the least fatiguing and I can trust in their truthfulness to the source. Some other headphones feel like they have the equivalent of a loudness swith in crappy consumer audio products that provides artificial excitement, or car radios with the treble and bass turned up to +11. Sure that adds artificial excitement but its fatiguing and coloring the sound (in my opinion) negatively. I know this is supposed to be a biggest dissapointment thread so I'll stop here, but for people who like high end audio through speakers (more from Thiel, Wilson Audio, Talon, Von Schweikert than Klipsch or Yamaha) would *probably* prefer the Senn sound signature. It has a similar purist bent.