A reciever is made up of a preamp, processor, amp, and tuner. The tuner obviously gives you a radio source, the preamp lets you switch between sources, the processor deals with effects/decoding/equalizing, and the amp of course makes it loud.
If you use the preouts on your HK you should only be bypassing the built in amp but still getting all the other functionality from it. If you are basically trying to get more volume, that should be an effective way to do it. It will also mean that anything else you decide to listen to will benefit from the additional power if you crank it up.
