Has anyone tried a 5-string upright with a low E and a high C string? If so, what string combination did you use? Thanks!

I've never seen an *acoustic* upright strung with a high C, which isn't to say that it can't be done. Finding strings might be a problem; solo sets aren't designed to be tuned above A, and lighter guage cello strings won't accomodate a 40-42" scale. Try Ned Steinberger or Martin Clevinger for strings. I know they make electric uprights with high Cs.

Hi.
I know of three brands of Hi-C bass strings:
Pirastro Permanent
Pirastro Flexocor
Pirastro Jazzer

There may be others, but I didn't find them yet. :)

I have seen more than one upright bass with the high c tuning at Bluegrass festivals. this is quite common they also make a low B string these are not as common.