Watch the The Thumb: 2 online guitar lesson by Muriel Anderson from 1-2-3 Fingerstyle Guitar
Here are a couple of ways to have a crisp, detached sound in the bass.
The first way, muffling the bass, is typical of the playing style of Chet Atkins and Merle Travis. The technique was first developed to prevent the bass notes from feeding back on an electric guitar, and the sound became stylistic.
The second way is a technique I learned from Leo Kottke. This is unique, in that it allows the hand to be over the soundhole, where you can get a greater variety of tone from the melody notes, and still keep the bass notes from ringing too long.