Since you are hooking up your DC with normal RCA connections, you'll also need to hook up your receiver's Monitor Out connection with the RCA connection to another Video Input on your TV. So if your Monitor Out S-Video is hooked up to Video 1 on your TV, hook up the Monitor Out RCA connection to Video 2 on your TV and use that when you want to play your DC or watch tapes on your VCR.
Or, you can hook up your DC to your receiver with an S-Video cable and use the RCA connections for the audio and you'll be just fine with your current setup, however, since your VCR isnt hooked up to your receiver using S-Video cables, you'll still need to hook up your Monitor Out RCA connection to your TV.
If your TV doesnt have another set of Video Inputs, then you might need to buy a box of some sort to let you switch video inputs. Dont try hooking up the Monitor Out S-Video and RCA connections to the same Video Input, it wont work.
About the speaker volume, there should be a setting on your receiver that designates your speakers are "small" or "large". I dont know if its a menu setting or a knob, I think its most likely a menu setting. This might affect the volume coming out of your speakers since you feel that they arent quite loud enough.
--Ben