Recommendation: Complete my studio monitor system with a center channel

Agree a single HS8 would be your best option.

Your best match would be to find yourself a single HS8.

It really depends on what you want. When you say twice you really are in love with that Yamaha Natural Sound, that is a trademark not actual natural sound. Furthest thing from it. But there was a time I was in love with JBL sound, and solid state, and looking back there were several times I rejected vastly superior components simply because they didn't meet my preconceived notions of what should be. 

So if you really want to bask in that YNS, then what you want is a center channel very close in sound character to your L/R speakers. Whatever they are. I can't get into it enough to recommend one. 

Because when set up right the center image is so rock solid there are few things more superfluous than a center channel. I had a guy one time so convinced I had a center channel he went and lifted up the blanket I had covering the TV, so sure he was that there was a speaker hiding under the blanket. Walked right up to it convinced the whole time it was hiding a speaker. Think about that for a minute. That is how little use there is for a center channel speaker. 

Unless of course you are convinced you need one anyway. In which case, your cave, your system, your choice. Go for it. You know what you like, and that is the First Audiophile Commandment: Know Thyself. 

mrdoug1, I think you would be better off talking to the theater crowd. Most of us think in two channels only.

I have always tried to keep the front three (Left, Right and Center) voiced the same on a surround set up as they all work together to create the front sound stage. Going with a mismatch center compared to the right and left front speakers seems like a bad idea to me.

If Yamaha doesn't make a matching center for those studio monitors, which I assume is the case, you have a couple options. You can look for a Yamaha center that uses the same or similar drivers to try and get a similar sound out of the front 3. The other option is to get a single HS8 studio monitor to use as a center so the front three are all identical. This is probably the option I would personally shoot for.