You can find the DT48 here:
Beyerdynamic | DT48E - Stereo Headphones - 25 Ohms | 404306
Their emailed price is $336, but it says they're out of stock. They used to be a special order item, so they might be able to order one from the factory for you. Also keep an eye on eBay since they used to go under $100 not too long ago.
The K-701 isn't bad for classical and acoustic material, but I think the K-501 beats it handily. Unfortunately, the K-501 is out of production, but I've seen them sell between $100-$150 used. They're somewhat common, so you will see them turn up pretty often.
Another out of production headphone that's great for acoustic and classical material is the AKG K-240DF. I think there's a pair on eBay right now. They're not hugely sought after, so you can often pick them up under $100.
One easily found headphone that's in production is the Sennheiser HD-600. I like the three above better, but the HD-600 is versatile and an excellent all around headphone. Classical and acoustic music is very enjoyable with it, though not quite as special as the DT48, K-501 and K-240DF.
And not to make you spend more money (
), but tube amps tend to have terrific synergy with acoustic and classical music. The natural mids are a sure match for glowing triodes.