What do you mean by "just bought"

Have you had time to wrap your head around what's going on? To work on systen set up? To try new/different cables?

First, do no harm. And keep it simple. Integrated amps have lots of advantages over component setups. If you need more grunt, forgoe the grunt and get easier to drive speakers or get a better amplifier and use the Yamaha as a preamp.

Adding a preamp won't improve audio quality. If you are unhappy with your sound, it is the integrated amp. I don't like criticizing people's choices in equipment as everyone has their own opinion and likes and dislikes. I really don't like Yamaha equipment and for the same money, would have purchased NAD over Yamaha any day. If you can return the Yamaha, it might help.

My experience with Yamaha integrated amps has left me with the impression that they are not good-sounding at all. You should replace yours with a high-quality integrated amplifier. Good separate preamps cost at least $1000 in any case.

For a very good sounding integrated amplifier, get the Music Hall 15.3. It will sound MUCH better than what you have IMO, and costs $549 from Audio Advisor or Music Direct. It should be a perfect match with your speakers.

Another good one is the NAD C326BEE, which is the same price.