Not all Sennheiser products are uniform in their build quality - they have a huge gradient across model ranges and prices. You're putting far too much weight on that single experience with what was probably one of their cheapest models. It does not inform all of their products.
If such build quality concerns bug you, avoid Koss (they tend to use very light-weight plastics on the cheaper units) and Grado (which can be inconsistent in their build (at any price point)). The higher end Sennheisers are uniform and well put together.
Sound-wise, the Grado cans are very "closed in" sounding, despite being open, due to their design. The Koss dynamics are fairly polarizing and don't follow any rhyme or reason in their overall sound signature. Aside from the (discontinued) A/250, I'm not aware of an open, around-ear, model that many people gravitate towards. Sennheiser's 500 and 600 series (and the 280) will not be "muddy" but retain a relatively smooth/laid-back sound, instead of the aggressive/harsh sound that you'll get from Beyerdynamic (let me qualify: I'm not a Beyer fan) or (potentially) Grado.
AKG is worth consideration, but they have nothing in your price range. Same goes for Sony, Kenwood, STAX, and so on.
If you want the thing to feel like it could survive a nuclear war, Ultrasone is one of your best options - Beyerdynamic is probably next closest on that list (but I don't think the DT880 will fit into your price range).
So the HD 280 or HD 555/558 are worth considering, is my point.
I'd also suggest the Bose AE2, but the build quality is not the highest; they have a good sound-stage though, and are relatively balanced (especially in their price range). Not perfect, but again, nothing at this level really will be (IME you either sacrifice build quality or sound quality, and when it comes to sound quality, it's usually at one extreme of the FR).