The answer is no. Your Onkyo 705 already has a pre-amp. In fact, it has a pre-amp, amp, and tuner all in one box. The pre-amp section is what allows you to switch between different inputs (e.g. CD, DVD, and FM). The pre-amp section also adjusts the sound levels, does digital to analog conversion, and all the other sound processing.

If you did want to get an outboard pre-amp, you would also need to buy a separate amp or amps.