I agree generally with your comments about optical vs USB out of a MacBook. In my case though, I can only use optical for Hi Rez files as my RWA Isabellina has a second DAC chip for Hi Rez but no USB implementation.
I haven't replaced mine. While I liked them to some extent for their smooth sound signature and wide soundstage, my B&W P5s are my preferred bedside rig and travel option, out of an iPod - unamped. I am curious though about the new IE800.
I don't think that they will send you a replacement left driver, as I had a problem with the left driver on mine recently - reduced volume. If they can't fix it they may offer you the IE80 for a reduced price. In my case, this price is close to the price I paid for my IE8 three years ago, and I didn't particularly want the IE80, so I was not interested.
My musical tastes are similar to the OP's and I have owned both these headphones. My HD650s were recabled with a SAA Equinox cable. I have also owned the HD600.
While I think there are better options out there at higher prices for your musical preferences, the street prices on these two headphones make them relative bargains. Of the two, I would take the HD650s, or for a more neutral sound signature, with less bass, the HD600 would be a better choice IMO.
Like you, I enjoy all things classical, and I have owned a number of different headphones. If you have a strong preference for detail and neutrality in a sound signature, then I would recomend that you consider buying a used Stax system. In the long run it will save you money if classical music is your preference.
If you are going the dynamic route I would recommend trying the Shure headphones in addition to the headphones mentioned by others. They are easy to drive out...
I have owned them both and sold them both. The answer to your question is "it depends". If you prefer a very smoooth sound signature that is on the warm side of neutral, with bass emphasis and laid back ... It has to be the HD650. If you prefer a more neutral 'monitoring' sound signature, then you will likely prefer the HD600.