I couldn't find specs on the TX-NR707 but the SR707 has a power output rating of 100 wpc. I am sure that rating is based on 1 or 2 channels driven and goes down as more channels start using power. The 7200 will put out 300 watts to every channel if pushed to the wall with an input of 1.43 volts from the processor. Since the processor is only capable of outputting 1 volt the amp is not getting all the signal it needs to provide the rated output. I would guess that since your are providing a little more than 2/3 of the needed input you are getting about 2/3 of the rated output. I would estimate that the 7200 is putting out about 200 wpc into 4 ohms. That extra power will only result in a 3 db improvement in sound pressure level at best. Your speakers appear to be 4 ohm impedance so they are a bit harder to drive but will present no issue for the 7200 with the full input voltage. What you do gain from the 7200 is headroom. Also by feeding the 7200 only 2/3 of its rated input I would expect you will never have that amp clip at any volume level even though you may not notice a big increase in available spl.

As far as a power conditioner, you will need one capable of providing about 2000 watts of power which is what the amp could POSSIBLY draw in worst case. However...it is highly unlikely you will ever push that amp anywhere near its limits. The power supply in the amp does not need conditioning unless you live in an area where the voltage sags or peaks frequently. What you really need is a good surge suppressor so lightning or other grid failures don't take out your expensive gear. I use power conditioners to provide superb suppression but there are many good suppressors available that don't cost a lot and provide a lot of protection. If you get one make sure it gives you some type of warning when it is no longer capable of surge protection. Some turn on a light but the best ones just prevent power output if the device can't do it's job protecting you stuff.
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Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner

TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv


Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner