There were new TTS voices for VoiceOver when iOS 9 was released. Samantha used to be the only english VO voice available, also including Alex. But Apple has now added the Siri Male and Siri Female voices, each of them with enhanced quality voices included.
To change the TTS voice of VoiceOver, go to
Settings > General > Accessibility > VoiceOver > Speech.
Select the Default Dialect option.
Now, you can flick right until you hear "Siri Male (Default)." This is the default voice of Siri Male. If you flick right again, you will hear "Siri Male (Enhanced)." This is the Enhanced quality voice for Siri Male. Flicking right again, the same things will be found with Siri Female.
Note: You will need to download the Enhanced voices that need some of your storage.
While you are using these voices, you may want to switch back and forth without going to Settings and all of that.
Go to Settings > General > Accessibility > VoiceOver > Speech > Add New Language.
Choose the language you want to put in the language rotor, and double tap on it. You can then download TTS voices to the language rotor and use the regular rotor gestures to change them. Remember, the rotor gesture is where you turn your fingers on the screen like a knob.