How Low Can You Go?

``HOW much will I have to pay for a really good subwoofer?" It's a question ducked a dozen times a day in hi-fi stores around the country by salespeople. It's not that they don't know the answer; they just don't want to scare anyone off by quoting a realistic figure. The truth is that playing back low bass at high volume requires a large cone, a large cabinet and a high-power amplifier, none of which comes cheap.