Roughly a quarter-century ago, B.B. King said of Joe Bonamassa that "he hasn't even begun to scratch the surface."

It was an undeniable compliment to somebody not yet in his teens, but it was also a challenge - one that the blues-rock guitarist, singer, and songwriter apparently still takes to heart. Bonamassa continually scratches and scratches to get deeper.

His performance April 19 at the Adler Theatre will be one example, featuring a set with his acoustic band and another with his electric - both covering roughly 10 songs. The acoustic sets demonstrate that Bonamassa isn't content to skate by on instrumental virtuosity - unlike too many of his ace-guitarist peers. These shows require solid songs, nuance, and variety.

As he said in a phone interview last week, the two-set engagements are "very challenging vocally and on guitar, because you're essentially switching gears tune to tune."

Even better evidence of his range can be found in his recent discography. In the past two years alone, Bonamassa has put out the Driving Towards the Daylight studio album, live and studio releases with singer Beth Hart, the third and final album from the Black Country Communion super-group, a studio disc by the jazz-fusion Rock Candy Funk Party, Beacon Theatre: Live From New York, the live album An Acoustic Evening at the Vienna Opera House, and the four-disc Tour de Force: Live in London - documenting themed shows at four venues with different band lineups and more than 60 different songs. And he has a new studio album planned for fall release. (The old saw about the weather can be adapted for Bonamassa: If you don't like his latest record, just wait a few minutes.)