It sounds as if you're getting good advice given your priorities.
Personally, I value tonal purity and richness somewhat above sharp definition. To give a video analogy, I would be more concerned with the contrast ratio than with the number of megapixels. I think the audiophile community is still giving first priority to the sonic equivalent of megapixels.
What would a budget option be given these different priorities? It depends on your music, of course. Suppose it's classical. In that case, try vintage Sennheisers on Ebay. The HD 530 and HD 560 recently sold for $35 and $43 respectively and (confusingly) are later and reputedly better models than the famous HD 540.
You will then need a good source (CD player or DAC) and an amplifier. I have tinkered with equipment lying around. The HD 530, which I have, sounds dull just plugged in to a Sony boombox. It's not much better when you feed that boom-box into an amp; that Sony is just a poor CD player (i.e. a cheap DAC). I had heard that DVD players are bad for CDs, but out of curiosity, For a budget amp, consider the receiver-amplifier in an Onkyo mini system with Wide Range Amplifying Technology (or any good receiver-amplifier).
Note that these earlier Sennheisers have the p4x connector, which is different from the one for the HD 580/600/650. So it's not tempting to buy the Cardas upgrade cable or its rivals. (Buy replacement cables and cushions, which you'll probably need with these old headphones, from the Sennheiser USA store: resellers double the price.) The cable upgrade may not be necessary anyway. I don't know why it's important to the 580 and later, but perhaps because the bass interferes with higher tones, at least in the stock cable. The HD 560 and earlier don't have that deep bass and thus don't have the problem. (For the same reason, they won't satisfy bass-seekers. Neither will AKG k 702, by most accounts. The HD 530 does have plenty of bass for a cello.)
I imagine that the audiophiles have already fainted, but the result is surprisingly good. Listening to Mozart's Divertimento K. 563 performed by L'Archibudelli, you can hear the texture of the strings with great clarity, richness, and beauty. The same for a Bach Oboe Concerto, where the oboe is stunning. The sound is no doubt more blended in a symphony than you'd hear with an AKG k702 and a c 2c amplifier ($500-$600 combined cost) or even an Audio Technica in the ATH 700 (closed or open), which would be much cheaper than the 702 and c 2c. Sym-phony does mean blended sound, after all. Not that I wouldn't try the 702 and c2c if I could justify the expense.
