Daryl means speaker level inputs - many subs include binding post connectors for routing the speaker cables through the sub (see
this picture of the LFM-1 for an example). The result would be functionally the same as the sub/sat connections on your sub, except that they occur after the amp instead of between the processor and the amp. Your sub lacks these, however, so Daryl's arrangement wouldn't work for you.
I haven't ever tried connecting both unbalanced and balanced outputs on the 990 simultaneously to know if it would work. An e-mail or call to Outlaw could get that answer. Obviously, you could use the 990's unbalanced pre-amp outputs and connect them through the sub's amp and then to the 7500 as shown on page 11 of the PDF file you linked to, but that'll mean annoyingly long interconnects as well as not being able to use the balanced connection between 990 and 7500. Either way, however, you'll need to make several changes at the 990 to make use of this mode (these will look familiar to Daryl's setup): set the sub to "off", set the mains to "large", and use the sub's low-pass crossover adjustment to prevent the sub from extending into the territory that the mains are capable of reproducing. With no crossover upstream of the mains, they'll be trying to run full range.
Here's a third option to consider: your sub's low-pass crossover doesn't have an option for bypass, just a max setting of 150Hz. On the bright side, that means that even with the 990's analog bypass mode engaged, your sub won't ever reproduce data higher than 150Hz. With this low-pass crossover behavior in mind, you might leave your cabling as it is now and experiment with two things (either separately or in conjunction with each other): lowering the sub's low-pass crossover setting to reduce the area of overlap (maybe ~100Hz), and adjusting the 990's 2-channel sub offset down some.