I have a song in my score for a pizz string bass with a very wide range. It goes up to a G# below middle C (actual sound, not the written part). The orchestration books I own put the practical upper limit of a string bass range to the D below middle C (actual sound). The lowest note is two octaves below that. It's a moderate waltz, with a nice walking bass line that periodically goes higher and lower than what you'd expect (which is part of what gives it interest and charm).
I've tried transposing down an octave, but it gets too muddy. If I take it up an octave and use the cello, it overlaps the treble voices and muddies them. Other keys will put it in a difficult range for the singer or the other instruments. I've tried to split the part between the bass and cello, but there are no good places to switch instruments without ruining the fluidity of the line.
I have two keyboards in the pit. One is playing the harp part. I can play the bass line on this as well and it doesn't sound half bad. Or I can have the second keyboard play a string bass patch. But I'd rather have the bass player play it, because he can add vibrato, slides, and other sutble effects which I think would suit this piece nicely. So, my question is: how difficult is it for a bass player to get the higher notes in this range - and does the quality of the sound suffer (or change enough where the switch might be jarring)?