If you're still looking for that freaked-out gut sound, forget it. Something else is happening. The latest word from the Underground is "Soul." Soul is cooler. Less wiggy. More music power. Guys with the talented heads are joining up. Jazz buffs are coming in. Don't mistake it for a "new sound." It's the New Music. The blow-your-mind, "acid" sound of the old generation of amps is out of it now. More and more, the soul amp in the Underground is getting to be Gibson?cooler, soulful, able even to handle a jazz figure. It's emerging. What if you're caught without it?