Here's the deal on the 990's second zone: think of it as an additional analog stereo pre-amp tucked inside the Model 990. If you have sources that you want to use with the second zone, you'll need to make sure you run stereo analog cables from the source to the 990 - even if you are using a digital connection under normal circumstances. Then you'll need to have some form of amplification for the second zone, connected between the second zone output and the speakers in question (just like the 7500 is connected between the main pre-amp outputs and your main speakers).

There are a number of options for that second zone amplification, depending on the type and number of speakers in question. There are distribution amps intended for whole-house audio setups (although they can get a bit pricey - a Model 7075 would probably be more cost effective than some of the more readily-available options), there are used two-channel amps on places like Audiogon, there are old stereo receivers from eBay or yard sales, and there's even the Sonic Impact T-Amp if your load isn't too difficult to drive.

The T-Amp is a little digital amp (something like 10Wx2, although some places claim it's closer to 15Wx2) that can be had online from places like Parts Express . The T-Amp doesn't have a tremendous amount of power (if you have two sets of speakers, I'd consider getting two amps and split the second zone output rather than trying to double-up on it), the volume control is apparently pretty cheap (the solution to that is simply to turn the amp all the way up so the volume control isn't messing with the signal - which works fine for this application since the 990's second zone has a volume control), and you'll need to pick up an AC adapter to power it (assuming you don't already have one that'll work). All those qualifications fall away when you consider the T-Amp's rather diminuitive $30 price tag, and the tiny power consumption makes the absence of a trigger less of an issue (just leave them turned on all the time).