Haha, I did research to learn that double flat 5th terminology ;) Simply a 5th, but flat, twice, which makes it a 4th, but with no 5th being played.
So I actually just got sent the music score. Turns out we were both wrong. It is made up of 9 parts officially (at least in the vocal version, not this Cello version). We have:
Solo: F5
Tenor: F5
Lead: Eb5
Bb4
Baritone: Bb3
G3
Bass: Eb3
Bb2
Bass Extra: Eb2
This is of course for the vocal version. I think the PianoGuys version doesn't have those extra low bass notes in it though. Oh and the Solo is playing the same parts as the Tenor, though he is singing some different words and there are a few bits sung differently in the vocal version.
Of course I am looking at a different score so it's entirely possible that these are not the notes used on that video! But I did play them on my iPhone piano thing and it did seem to fit.
Just skimming through the sheet music and some of the bass notes go down to a C2!!! What insane Bass singer does that?!?!