That's certainly an option, but not the one I would likely go with since you have a quality pre-amp with 3-zones already. Adding a secondary pre-amp/amp on top of your nice multi-zone preamp seems somewhat redundant. Also, you will need to feed that Yamaha from the Zone 3 output of the preamp, or split all your audio connections to both products. If you connect it from Zone 3 of the Marantz, then you will need to power on the Yamaha, make sure it is on the correct source, and power on Z3 of the Marantz, and pick your source. It's going to be another step every time you use it.
If you want to do it on the cheap (but good quality), and have solid volume control in both areas, you certainly don't need another pre-amp to get there.
Something like this:
http://www.ebay.com/itm/Sonance-Sonamp-260x3-mkII-Power-Amplifier-/282467825936?hash=item41c464f910:g:M8YAAOSwYlRZEMN~
Will give you amplification for up to 3 stereo zones (same source)
Plus, you can add a in-rack volume control option, like this:
http://www.ebay.com/itm/2-Pair-Speaker-Selector-Switch-with-volume-level-control-2-Amp-capability-100W-/391538340476?hash=item5b2980f27c:g:Pm4AAOSwHoFXuGIS
That way, you can run the speakers separately, and maintain separate volume control with the touch of a button to turn them on/off. While no local volume control, you would still have individual volume control.
It's a couple hundred less, and gives you greater flexibility within your setup overall.
Up to you though.