After considering the potential configurations, I'm definitely leaning toward the secondary-amplifier option. The two-volume-control option has an advantage in that you won't need to provide power for a secondary amplifier (if that would be an issue), but it would mean that you'd have to turn the gain on this channel way up so that the patio speakers could potentially play louder, and I'm concerned about potential surround channel headroom issues that could result when playing movies--not in terms of power, but digital headroom and how high a voltage your receiver can output since the surround speakers would be operated at 16 ohms impedance with attenuation from the volume control on top of that (it is possible for your receiver to be short on voltage even if it still has power to spare). You could compensate for this by adjusting the surround channel gain (in the receiver) and both volume controls as necessary, but that seems rather inconvenient.
The secondary-amplifier option is safer in the sense that it should not affect the sound or capabilities of your surround speakers, at least, as the LOC couples with the speaker cables at a very high impedance. My main concern is how well the line-level output of the LOC will work with the amplifier, and whether the latter will have sufficient gain when the surround speakers are playing at low volume (both the volume of the receiver and the amp will determine the ultimate volume of the patio speakers). I expect that everything will work out satisfactorily (as long as you're not expecting super-loud output, of course), but I can't truly know without trying it myself, and I don't have an LOC to play with.
As for sound quality, the LOC that I recommended above
is one of the best, if not the best, passive LOCs around (it's the best that I know of, anyway). It will slightly affect sound quality, but you may or may not even notice. Here is a measurement of its frequency response: http://tom-morrow-land.com/tests/navoneloc/
. Aside from the 3 dB peak at 80 Hz, it looks like it's good down to 50 Hz or so, and is nice and smooth in the midrange and treble. But if you want to play it safe in this regard as well as the interface with the amplifier, then you could use an active LOC instead, such as the AudioControl LC2i:
The drawbacks are cost and the fact that you'll need to supply it with power, which means even more equipment--it expects to be connected to a car battery, so you'll need something like this: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=320-312
. I guess you'd have to cut the 12 VDC output cable, determine which is + and which is -, and connect them to the LC2i.
You can place the LOC anywhere you want--whatever would be most convenient, which is probably wherever the amplifier is located. In this case, you'll need to connect a speaker cable in parallel with the surround speakers, and run it through the wall to the LOC, which is then connected to the amplifier with an RCA stereo cable.
By the way, LOCs are almost exclusively used in car audio, so forums dedicated to this subject should provide a lot more information on those than you're likely to find here. They probably haven't tried to use this equipment the way you would be using it, though, so in general there is very little precedence to work with, unfortunately. In your place, I'd definitely purchase and experiment with the equipment before ripping up any drywall.
As for which amplifier to use, the Dayton Audio DTA-100a
mentioned earlier seems to fit the bill pretty well, due to its compact size. I've confirmed now that it can be turned completely off by rotating the volume knob fully counterclockwise.
And as for speakers, there are numerous other threads--many quite recent, in fact--that have recommendations for outdoor speakers. In any case, here are some examples that seem quite nice: