Nothing wrong with using just 2 channels. In fact, by only having the L/R hooked up, you will be closer to the receiver's "rated power" than you would if you had all channels in use.
That said, the RX-V675 should have PLENTY of power. I blame either your speakers or perhaps how you have them placed + room acoustics. As stated above by 3db, we need a ton more info before we can really help you out that much. Tell us more about your room such as dimensions, furniture, etc. I know it sounds crazy, but that stuff can make a huge impact. I recently hooked up a system in a completely empty room and it didn't have much oomph. I told customer to finish moving in so I could finish calibrating everything and finalize setup. After he had his big couch and the rest of his furniture and stuff in, it majorly increased the gain in the room, and the sub and all speakers sounded WAY better and plenty powerful. If you have a 10" sub and puny speakers in a massive room, the results will typically not be great.
