This.LowF, You'd be better off adding a second 4x10, as a 1x15 wouldn't be able to handle as much power or move as much air as the 4x10 cab can. Adding a 15" speaker won't make your sound deeper, that's a common misconception. But there are folks in here who can more accurately explain why that is.
LowF the best match to your existing cab, is a second one. That said. I think you could get a lot more out of a cab upgrade, than by adding a second. Better to shoot for a really good used cab, than a bottom of the barrel new cab too.
