As to whether or not a single 10" sub in each tower will be sufficient, It will depend on how big of a room he has these speakers in. If the room is small to medium sized, then a pair of quality 10" subs may be ok, but, if the room is big, then I would assume that it would not be optimal.
Have you considered mounting a larger sub, say...a 15" on the sides of the 1099 enclosure a la Definitive Technology BP-2000 style instead of mounting on the front baffle? I am not sure of the cabinet specs for the 1099, but. I am sure that you could play with the volume in order to get where you need to go'