That was really well done on all 4 instruments, the marimbula sounds a lot like an upright bass, what a cool addition to an old time band!
In all of the mp3 audio recordings I've posted both on the Fiddle and Banjo Hangouts, I've used this marimbula instead of an upright bass. For these types of recordings, it certainly does well. I've even taken it to small jams, extended the "cello-pin" at its bottom so I can stand up to play it, and it very closely approximates the upright bass in a small jam. Much easier to haul around too!