Q. I have an older stereo system with bookshelf speakers. How do I upgrade this system to a home theater? What should I add next? — A.C.
A. Besides getting an A/V surround receiver with Dolby Digital/dts 5.1-channel decoding, you could start by getting a center-channel speaker and two surrounds, using your existing bookshelf speakers as the main front channels. Although you may have some tonal differences between the center and your older bookshelf speakers, you would still be able to enjoy full 5-channel home theater, albeit without the deep bass impact that a subwoofer would bring with the ".1" channel.
Depending on your budget, I would next add that all-important subwoofer, because part of the thrill of current DVD soundtracks are the deep bass music and sound effects.
Assuming you’ve budgeted for the DVD player and A/V receiver, another approach would be to retire your existing bookshelf speakers to the role of surround speakers, and, depending on the room size, replace the front left and right speakers with modern floor standing or bookshelf speakers.
You needn’t get a center channel immediately (although I consider it fairly essential) because the movie dialogue will be roughly centered between the main speakers so long as you don’t sit too far to one side. Later on, add the center channel, the subwoofer, and, if you want a more enveloping sound field, a pair of bipole/dipole multi-radiating surrounds.