The 170w 5803 will be able to handle them, no problem. I occasionally ran my older 125w Denon 1025 with my van 3aSigs and it was a good sounding system with a surprising amount of bass. The higher quality home theater receivers do sound pretty good, especially if the speaker is a little more gentle natured. The 5803 likely has a stout power supply designed to source all seven channels. If you are running only two channels this power supply should be able to drive the speakers with more authority.
If you ever want to put an amplifier on them the McCormack (disclaimer I have one for sale) amplifiers are a classic combination. Those really bring out the bass and are a match made in heaven. You can pick those up used for $600-900. See if it is possible for the 5803 to act as a preamp for the two main output channels while acting as a preamp and amp for the remaining surround channels. That might be a nice upgrade path as would an adcom 750 preamp which has a home theater bypass input (your denon would be the volume control for this input).
If you ever want to put an amplifier on them the McCormack (disclaimer I have one for sale) amplifiers are a classic combination. Those really bring out the bass and are a match made in heaven. You can pick those up used for $600-900. See if it is possible for the 5803 to act as a preamp for the two main output channels while acting as a preamp and amp for the remaining surround channels. That might be a nice upgrade path as would an adcom 750 preamp which has a home theater bypass input (your denon would be the volume control for this input).