Your comments have been very helpful. Thanks!
I like the cello idea. It seems like it would be much easier for them to play that note than it would be for the basses, and it would sound a little less harsh.
I think the oboe/bassoon loudness is definitely a factor of the playback (when it was performed live the balance sounded OK). What exactly do you mean when you say you were "rhythmically lost?"
I'll look for the collisions you mentioned. Those can be pretty annoying to look at.
Oh, and in the performance, the conductor actually changed the singing part and just told the basses to play a D instead. Apparently, most of the basses couldn't sing that low! I might instead write "optional: sing" or something like that.
Thanks for your comments! Now, time to edit...