You sure? The only review for the HD515 I can find says they have a muddy sound.
Relative to the HD555, maybe. If you're worried about it already, though, you should probably just look at the 555, because otherwise indecision and doubt are going to eat away at you if you buy it.
Honestly, I wouldn't know what "muddy" or "bright" entails nor would I put that much weight on the review. It's all subjective. You may be able to notice a difference when comparing headphones side-by-side
"Bright" headphones are typically characterized by a frequency response hump roughly between 3KHz and 7KHz, or a single FR peak somewhere in that area (usually around 5KHz). "Muddy" headphones are typically characterized by a frequency response hump somewhere between 120Hz and 250Hz and/or slow decay in that area (waterfall plots show this nicely). If something can't be explained in scientific terms, it's not worth explaining; if you personally can't explain something in scientific terms, that doesn't mean it's not worth investigating.
but if you have only one at hand you wouldn't care for anything else except for how much money you spent and now you can't pay for your bus fare. Then you start to wonder whether you should've gotten the one $10 cheaper.
This is not true, actually. Many people worry about whether or not they should have gotten the higher model, figuring that $5 or $15 or $50 isn't that much in the long run and they might be missing a lot. In the scheme of things, I doubt you'll find many people that say, six months later, "damn, I should have saved ten dollars and gotten something that people say is worse!"
Here's something to think about: video card manufacturers will downgrade their top-of-the-line cards and sell it at the mid-price-point just because they know they can sell more. What prevents headphone goonies from doing the same thing. What if the HD-0.01 actually uses the same diaphragm as the HD100000. Won't that be something. Afterall, it's more cost-efficient to manufacture a single diaphragm and put them on multiple headphone models as opposed to manufacturing unique diaphragm for each headphone model. Do you really think a HD101 is that much more different from HD100. I think not.
This is interesting speculation, but unfortunately it is also wrong, and you have made the mistake of drawing a conclusion from your unfounded assumptions.
Even headphones that use the same transducer can sound very different in different enclosures, but that's beside the point. You see, there is surprisingly little transducer sharing between headphones. It may seem counter-intuitive, but it's the way things are, as you would know if you favored evidence over speculation. Real-world examples: the HD201 is actually very different from the HD202, the HD565 and HD570 are completely different headphones, but the HD565 and HD580 are fairly similar and the HD580 and HD600 are nearly identical.