An easier to understand illustration of sound stage is just that the music sounds like it's coming from outside your head. With Grados, the music tends to sound like it's right in your head. Some people can place instruments with headphones that provide good sound stage, some can't. I'm in the second camp, but my favorite headphones all have good sound stage.
As for sub $150 headphones for classical. You might be able to get an AKG K501 for around that, maybe a little more. You may also be able to get a K400/K401/K500, but they're harder to find. You might also be able to find a Sennheiser HD580 for under $150. All of these need an amp. I use the headphone jack of a vintage speaker amp, which is more than adequate.
If you have a speaker amp, you might look for non Stax electrets, like Audio Technica ATH-7, ATH-8, AT-705, or AT-706, or Signet TK33 or TK44. If you go this way, make sure you buy a headphone an transformer box. You connect the transformer box to the speaker terminals and then plug the headphones into the box.
I know people recommend the current lineup of AT phones as well, but I've never heard them.
As the OP recently stated, the Yamaha HP-1/YH-1 sound very good with some very easy, reversible mods. They have very little sound stage and are hard to drive, but can usually be had for under $100. Head over to
http://www.head-fi.org/forums/f4/ort...oundup-111193/ for more information.