My good buddy.
Hey dude,
#1. NO, you won't hear a difference, the main benefit is only a small increase in the gain. But in your case, your 705 should already have a sufficiant gain from it's subwoofer output. So, all right, save your money, and buy a six-pack (beers) instead.

And just follow the instructions for your sub (Left RCA jacket only).
#2. Yeah, your sub got a 3-way power switch. But my advice is to select ON, especially just before you run the Audyssey Room EQ calibration, so your sub won't wake up too late when the test signals reach it. Comprende?
After that, select any position that you like.
#3. On your receiver, the Default position for your speaker's impedance is 6-ohm. You're good there, just leave it at that.
And for the x-over points for your Energy speakers, 120hz seems to be a very good choice.
#4. Is there a button on the front panel of the 705 that says "HDMI OUT"?
If yes, when you are playing a blu-ray disc (1080p), just press that button once, twice,... and see what happens.
Now, the "HDMI Audio" has to be set at OFF, because at ON it means that you want to listen to the audio from your TV speakers! Do you now?
That's what I thought also. So, set it to OFF. period! Then, the sound will be output through your receiver HDMI output to your speakers, exactly what you want. If you don't believe me, just try it set to ON. You'll hear nothing anymore coming from your receiver!

* Just read that section in your manual, it is clearly explained black on white.
** In your 705 manual, it is indicated near the end, on the "Advanced Setup", where it says: "Tuner", then "Analog Multich", and finally "HDMI", and right below "HDMI Audio". It's just before the section for "Zone 2 and Zone 3",
about 15 pages or so before the end of your manual.
*** And YES, it should be at OFF, because you are using the HDMI version 1.3 cables to get the high resolution audio codecs for your surround sound.
Like I said, ON is for your TV little tiny speakers. I know it is confusing (a lot of people get screw right there), but that's the way it goes. Don't ask me why, I did not build these Onkyo receivers.
#5. * I am who I am, and I always give the maximum of my time and energy each and every time, that's why I choose my signature name, or whatever you want to call it, LOTR.

And I'm very proud of that name. Suits me just fine.
>>> And YEAH, do read your 705 bible. Great idea, take your time, and grasp the meaning of what you need to grasp.
~ I bet you're starting to enjoy your new system, you pick all the right ingredients. Now, it's just a question of reading, learning all the setups, etc., and finally, FINE TUNING for the rest of your life (ETERNITY).



Then, you become an addict of audio (AUDIOHOLIC

), and FOR LIFE too.
Enjoy, while you're ALIVE.
Love you Gamblet,
Bob