Welcome to the forum!
What receiver do you have? If you have a surround sound receiver with a set of pre-amp outputs for the front channels, this is how I'd do it:
1. Connect the TV output to the receiver (like you have it)
2. Connect the amplifier to the receiver using the pre-amp outputs on the receiver and the RCA inputs on the amp
3. Connect the front speakers to the amplifier
4. Connect the rear speakers to the receiver
5. Set the receiver up to use a "phantom" center channel (which means that it will fake one using the front two speakers)
6. Turn a surround sound mode on so that all of the speakers will get used
If the receiver has it, you can also run an auto-setup routine (which would take care of step 5).
If your receiver isn't a surround sound receiver, you can still do this. The best way that I can think of is to use a 3.5mm-to-RCA splitter, then a second pair of RCA splitters - this is so that you can connect the headphone jack on the TV to a pair of RCA inputs on the receiver and to a pair of RCA inputs on the amp. This way, the speakers connected to the receiver and the speakers connected to the amp will be controlled by the TV's volume control. If you use the optical output for the receiver and the headphone jack output for the amp, then only the amp will be controlled by the TV's volume (the optical output isn't affected by the TV's volume setting), and you'd need to change volume on the TV and the receiver to get things to match.