From what I have read and learned from others who import altums (which you already probably know but I will say anyway), get that pH as LOW as possible. Where the altums are typically collected has basically NO bacteria count. Often if pH/hardness is even on the soft side, the angels react to the beneficial bacteria as some fish do to a cycling tank- i.e. clamped fins, extra mucous production, sitting on the bottom/lethargy (which can rub off their pelvic fins), and loss of appetite. This is of course most true for fish that are newly imported, rather than those that have already been acclimated.
Edit: if you can, I would pre-treat your water change water with a peat bomb (tub set up with tons of peat granules/leaves/cones etc) so that you can match things as closely as possible.