How to use my old B&W's with my new big screen??

First congrats on the new toy!! What you could do ..depending just how hard you feel you need the push the B&Ws? Is pick yourself up a receiver capable of the newest codecs. Denon,Marantz,Yamaha or Onkyo will do the trick.

Any of them connected to your speakers, will sound much better than the TV's! Grab yourself a optical cable and use it from the TV to the receiver. If you have cable and a cable box with digital output. Connect your sound from the cable box to the receiver using a optical or coax digital cable(this depends on the cable boxe's digital output).From there you can expand the system to your own needs.

By the way, over the air digital channels have great sound! You'd be surprise just how good prime time TV shows sound if you go the route using your inboard OTA digital tuner.

I agree Stanwal about the KISS approach with audiophile audio, unfortunately that particular KISS approach doesn't allow digital sound reproduction. You're missing a good portion of sound running analog two channel from a TV capable of so much more. This is where a receiver will excel without a doubt. Most prime time shows are broadcast in Dolby Digital.

KISS equals "Keep it Simple, Stupid" which IMO doesn't apply when you're setting up a HT system.