You will lose Audyssey Multi EQ XT as the 806 only has Multi EQ. Power will be very similar and something you wouldn't hear much of a difference on (MAYBE 2db increase). The 806 does have Dynamic Volume and Dynamic EQ.
806 has 2 more HDMI inputs which may be nice, but you may not currently or in the near future need. It also can upconvert to 1080p, but if you use HDMI on your 705 the signal is untouched (believe the same for the 806). This is really a great feature for sources like the Wii, DVD player.
Personally I wouldn't do the deal. I only use 2 HDMI inputs so 5 would be insane. I wouldn't pay for the other features (not $300 worth of "upgrades" to me). The Sound Quality will be the same. Features are the difference and you may not use any of them.
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