The Marshall JCM 900 may be well known as a guitar amp but it also works well for bass.
When you want to use the amp for bassapplication make sure you use speakercabinet which uses bass speakers.
Marshall is renowed for its distortion sound, you may wonder how does this work then for the bassguitar? Most bassplayers like to play with a clear sound.
The JCM 900 is a flexible amp in terms of tones, when you want to use the amp for bass you may want to dial in less treble. Boosting the midcontrol will give the amp more power, also add as much bass as you want.
The result? A mellow tone which can be as loud as you want. It may not be as deep as what people get from a traditional bassamp, but the sound is good enough to use in a bandsituation or for recording.
Give it a try to see what you think.
Happy playing and hope to see you soon again,