How do I connect my yamaha receiver to the 990?
I only want the amp part of the receiver.
Most receivers do not provide a direct connection to the internal power amps. There is a workaround, assuming that you have a multichannel (5.1 channel) analog input on the Yamaha receiver. It's not optimal, but we can make it work until you buy a power amp.
You will connect the unbalanced pre-amp outputs of the Model 990 to the left, right, center, left surround, and right surround channels of the multichannel analog input on the Yamaha. Once you do that, you will need to set the Yamaha receiver to that input and set the receiver's volume to a setting (probably a somewhat high setting) and leave it there. Once you do that, you will not make any changes to the receiver's input or volume. You will then set up the Model 990 as normally, as outlined in the user manual. It may take some tweaking of the receiver's volume control to find a good setting to use.
When i do get the power amp, does it have to be 5 channel?
You need one channel for each speaker you have (except for the subwoofer, which is typically going to be self-powered and can connect directly to the 990's subwoofer output).