I'm having the same problem, but it's more widespread than just Google Maps & Nav. It started when my Note 4 was reading all my calendar alerts out loud in an obnoxious, tinny female voice. It took a lot of digging, but I finally found where to disable that, but while I was mucking about in the TTS settings, I started playing with the various voice options. I have downloaded high quality voice files for both the Google and Samsung TTS engines, and made every possible adjustment to set them as default, but I have yet to see a single app on this platform actually respect the defaults I specify.
I know the HQ voice I downloaded is set correctly, because I set the system language to English (UK), installed the English (UK) male and female voice files, and when I set one, the TTS settings page tells me the language I chose is "fully supported", and the playback sample confirms it is applied correctly.
Still, Google Now, Navigation, and the Calendar app insist on using their own low quality defaults, no matter how many times I reset their data, wipe the caches, or reboot the device. As a final straw, I uninstalled all the Samsung TTS updates and HQ voice files, force-stopped, and disabled the engine in the app manager, and the calendar alerts STILL use the Samsung default. How the holy EFF is that even possible???
This is entirely too much brain damage for such a simple feature. If there is an entire settings sub-menu dedicated to assigning a default TTS engine, then every flippin' app that offers TTS output had better flippin' USE IT!