With the 990 as is, or with an SMS-1 or similar item involved, one could go through the setup procedure for any number of positions, writing down the results after each one. This might be painstaking, I know, but you’re an enthusiast, right? If I had a couch with three primary seating positions, I’d average those three results at adult seating height and enter the average values manually. They should be fairly close to begin with. If I had a couple other seating positions substantially different from my primary area, I wouldn’t create a poor listening experience for the primary positions just to make it better for the outlaying seats.

If a problem existed, such as much more bass response for seating near a wall of the room when compared to other positions, I’d might add room treatment (bass traps?) then go through the averaging setup again for the primary seats. One might still check the values obtained for primary versus secondary positions to see if room treatment helped create less of a differential, but I'd still favor an average of the primary seating area.