Units registering to spa9000, needs to know what the 'proxy listen port' and 'phone ext password' on the spa9000 is set to.
By default the 'proxy listen port' is 6060 and 'phone ext password' is blank.
These parameters are in the SIP tab of the spa9000.
So by default, I would be putting in the "192.168.1.200:6060" for the proxy parameter of the spa8800. And no password if the 'phone ext password' is blank. The userID should be the ext number that you want the spa8800 to be.