I use the 701s with a Woo Audio WA6. The 701s will work with most headphone jacks but they'll just sound way better with a purpose-built amp. The 701s, more than most headphones require a good bit of power and a really high quality amp; otherwise, they can sound harsh. Even an inexpensive headphone amp can sound poor with the 701s. If you want to try your Yamaha's jack you might consider the 501s or the Sennheiser 600s or 650s, which are way more forgiving than the AKGs. Another option is to buy a used Woo off head-fi.org or Audiongon.com so that you can get your money back out if you don't like it.

Dave

It is absolutely worth getting a dedicated amp. There are quite a few less expensive good amps. Go take a look at Head-Fi - lots of good info.

One to consider is Ray Samuels Audio Emmeline SR-71. Very nice.

I disagree with Dave that the Sennheisers are more forgiving - but they are different.

I, too, listen primarily to classical.

Bob2Bob,
Like yourself, 90% of my listening is to orchestral music also. I just picked up a new pair of the AKG K701's. I thought they sounded good right out of the box. I'm using the headphone output on my Denon DRA-395 Stereo receiver. I am having excellent results with this combination. These K701's like my K240S seem like an easy load to drive. In the past, with difficult to drive headphones, I'd use the headphone output from my NAD preamp. This preamp sends it's full power to it's line outputs and headphone jack as well. I think it is something like 15v. That NAD sounded better than most headphone amps I tried.
Try your Yamaha first, you may decide that's all you need.
Good Luck, and let me know what you think of the K701's.
BB