I am still putting my HT together. Do I need a sub with built-in X-over or can I use the 990 to feed the sub bass only? If so, then I can build a sub enclosure and use a channel from a 7700 or 7500 to power the sub. Has anyone done this?

The typical advise when using a subwoofer with the 990 (or any other processor or receiver with good bass management) is to bypass the subwoofer's crossover, so you do not need your sub to have a built-in crossover.

The 7700 or 7500 could drive a passive subwoofer, but when building your own sub you'll want to figure out what sort of power requirements the sub may have. Subs often operate at a 4-ohm load, under which conditions you would be getting a pretty respectable 300W of output from either of those amps. DIY sub makers could probably offer some thoughts on whether that would be enough power for a specific design or not.

I have a home made sub encloser with two 12" infinity woofers. They are 4 ohm rated. I was running them with a 2 channel adcom amp for years. After receiving my 990/7700 combo I decided to try using 2 of the 7700 amps channels to drive the subs. It works well and sounds better then the adcom did. More power from the outlaw amp. The 2 amp channels driving the subs do tend to get hot, but it has not shut down. The only thing I don't like is not having a dedicated sub crossover level control in the 990. My center crossover is the highest at 100htz, so I assume thats also my sub crossover?

If you are running your speakers as Small, then the crossover setting you chose in the 990 is the crossover used for your subwoofer. I'm assuming of course you are using the 2 subwoofer output jacks on the 990 to feed those 2 channels on your 7700. The highest crossover setting to the sub is only for that particular speaker(center). The other speakers bass information will crossover to the sub at the crossover you chose for them. This is the advantage of having quad crossover settings.

I was thinking about placing a 7500 at the rear of the room and using 4 channels for the surround and one channel for the sub, which is also at the rear. As I understand it, I can feed the sub with the balanced output and use the 990 to set the x-over independent of the settings for the other channels. Is that correct?

However, if I use a plate amp to drive the sub, I will give up balanced connection (I can't find a plate amp with balanced input) but I will gain the ability to adjust the phase of the sub.

Isn't having phase capability important?

No, the sub crossover settings are derived from the speaker x-over settings in the 990. There is no independent sub crossover setting. To direct only the surround channels to the sub, the Front speakers would all have to be set to Large.

The sub's signal is assembled from several sources. First, there's the LFE track - if the source has one. Then each small channel is divided into two signals at its chosen crossover point and the low frequency portions of each is sent to the sub.

Phase setting is important, but if the adjustment is purely 0 or 180 degrees you can do the same thing with the 7500 by reversing the positive and negative leads of the speaker cable at the amp.