I really like the Black Shadow C90 that comes stock in the F50 1x12 combo, though I occasionally play my F50 through a homemade 2x12 with a V30 and G12H30 in it...I usually situate myself to hear mostly the H30, which I prefer to the V30, though the blend of the two is quite nice, too. The cab is closed back and the bass is really big and deep with it. The H30 has kind of a crispy top end to it which I like.
Some really like the V30s with the F50, but it's not really what I'm looking for. It does tighten up the bottom end, but it don't care for the upper-mids that are so prominent with the V30.
I'm planning to get a Celestion Greenback to use with another amp here in the next month or two, and obviously I'll try it with the F50, but can't speak to that now. Though a guy on another forum who really likes greenbacks said he didn't like it with the F50.
One more thing, my 2x12 with the V30 and H30 has a tighter bottom end than does the C90 in the combo.