In one respect, not much has changed about this guitar-percussion duo since I wrote about them a couple years back: it's still about one guy banging sticks in time and another guy shortening and plucking strings in tune. But since then they've enhanced their finely focused interplay; they've slowly but steadily begun to gain a committed audience in their occasional appearances; and they've produced a quietly spectacular album (Loose Contact on Southport). That album surprised listeners with its impressive variety of timbres made by only two musicians, and also for the range of its 15 detailed and dissimilar compositions. But if the album surprised, the duo in concert may leave you dumbstruck--if only because they so graphically prove that the album's layered complexity involved no overdubs. Walker sprawls across his trap drums and arsenal of percussion instruments and makes telling use of a small supply of electronics; Onderdonk, who does most of the composing, switches among a classical, a steel-string, and a relatively rare eight-string Telecaster guitar, depending on the needs of each piece. The emphasis on creating each performance from spontaneously produced sounds--as opposed to computerizing the process, either in the studio or onstage--says much about virtuosity. But it says even more about maintaining the organic, elemental impulses that inspired this music in the first place. Friday and Saturday, 8:30 and 10:30 PM, Jazz Buffet, 2556 W. Diversey; 862-0055.