Just a thought, You might wish to add a second processing source to control your outdoor system.
a second preamp-amp/intergrated amp/receiver fed by your 990 zone 2 output would give you the ability to control the volume seperately and would provide a means for managing the bass in your outdoor zone. The drawbacks would be another unit to turn on and off and the cost, though 2 channel intergrated amps can be had for a reasonable sum, especially used, and it should replace the 2nd amp.