DownUnder73:like I mentioned I don't know much about separates, I have no idea what this quote means. So the RXV750 is like what you control the sound and source and the 2 Yammies MXD1 are you using that to connect to your speakers? I know I sound like an idiot. sorry
You may be a newby, but that doesn't make you an idiot. We all started somewhere.
Your AV receiver has several units inside it to do various things. There's a tuner to let you hear the radio, a pre-amp to boost incoming signals a bit, but mostly to let you choose which input you're listening to, like CDs, DVDs, the tuner, the TV, as well as whatever else you have connected to the inputs. There's also a volume control, something power amps and most CD players don't have.
After the signal goes through the pre-amp, it's sent to the power amp (a 7-channel amp in your particular receiver) to boost the signal to a level that the speakers can use. However, the receiver has pre-amp out connections, which let you bypass its built-in power amp in order to use an external, or separate, power amp.
When you add up all the components, including several decoders (for Dolby, DTS, and so on), that are in that receiver, all for a relatively low price, you can see how compromises in performance and quality might have to be made. That's where separates come in. You can get a good-quality pre-amp, connect it to a good-quality power amp, and so on, which will give you better sound.
Connecting a better-quality power amp to the receiver's pre-out connectors will give you clearer sound and possibly more power, if you choose a high-power amp. A power amp usually has only an on-off switch, since the pre-amp controls the volume and lets you select which input you want to use. When I got the MX-D1 power amp, the improvement in sound quality was very noticeable. There are several forum members using Yamaha AV receivers as pre-amps, controlling 2-, 5-, or 7-channel power amps, since those receivers seem to have decent pre-amps in them.
In my case, I used a stereo (2-channel) amp to drive my main speakers, while the receiver's power amp drives the center and surround speakers, since 2-channel music is important to me. The receiver has sufficient level adjustments to allow me to match the levels of all the speakers.
Recently, I upgraded the system further, bi-amping the main speakers, using one power amp to drive the mains' woofers and a second one to drive the mains' tweeters, but that's more complicated and expensive.