| Sustain is not usually affected by what amp you use but rather the bass and strings. The brighter and newer the strings - the more sustain (also roundwounds sustain longer than flatwounds or any other type) , together with the body of the bass and the bridge.
There is a school of thought that denser woods increase sustain and that a dense bridge - say solid brass will help, stringing through the body can also have a slight effect but not much. Generally, more expensive, "high-end" basses seem to have better natural sustain, but there are always exceptions.
There are artificial mean for sustain, but in a way this isn't natural sustain as such - for example compressors can be used to do thi, but will also bring up noise levels as they increase the volume of a dying note. The EBow has also bee mentioned recently as a way of getting infinite sustain, but this is probably only of use in a limited number of situations.
Anyway, a lot of this up for debate, but I think there would be general agreement, that you aren't going to find a knob on your amp that increases the sustain. You might be able to get feedback or a "boomy" sound, but this is usually undesirable on bass. | 
