Klipsch speakers are EXTREMELY efficient compared to the vast majority of speakers (including your Yamahas) so from that respect your results are normal. Efficiency is the ability of a speaker to turn watts into sound. The more efficient a speaker is the louder it will be with a given amount of power. There are two ways to correct it:

1. Get into the set up menu of the AV receiver and increase the volume (most likely labeled level) of the center relative to the left/right. If there isn't enough adjustment range to make them equal try reducing the level of the left/right in that same menu just a bit.

2. Use this as an excuse to buy the matching left/right Klipsch speakers. Not only will the volume be much, much closer to matching before correction. The tonality will be a better match so that as sounds pan from left to center to right it will much more seamless and coherent transition. One of the most important things in multichannel audio is to match the front three speakers.

Option two is the real solution with option one being more of a band-aid, temporary solution.

Sorry, I meant down of course.

Also just as a side note, more efficent does not mean better sound quality. Two different measures.