If my thinking is correct, the real benefit if Audyssey in this scenario is to set the distances on my speakers, right?
I think that's a fair statement, yes.
Using just sub-out 1 for all three subs seems like the distance measurement would be meaningful. Plus, REW/MSO does this for me, yes?
MSO can determine the sub distance in your AVR (or rather the amount you need to increase or decrease the sub distance from the value that was used in the measurement). This only works with MSO "subs + mains" configurations, though, not "sub only" configurations.
This brings to mind a kind of paradox that leads to the reason why the "sub only" configurations were added. Originally, only "subs + mains" configurations were available, which do optimum integration of mains and subs. For optimum integration of mains and subs, you need to run MSO
after Audyssey, because you need to have all the distance settings and EQ of the mains in place in order to get the sub distance and sub EQ in the crossover region correct. So you need to run Audyssey first. But, with multiple subs, the initial settings for individual sub delays and such may be way off, causing a large frequency response variation in the combined sub responses. Running Audyssey in this condition would cause Audyssey to make large response corrections using EQ that's common to all the subs. However, these problems are due to the subs not combining nicely, not necessarily due to some problem that should be fixed by applying aggressive common EQ. To get them to combine nicely, you want to run MSO first and Audyssey last.
So the "sub only" configuration type was created. The idea is to get the subs playing together nicely before running Audyssey, so Audyssey doesn't need to EQ the combined sub responses very much at all.
However, Audyssey tends to do a poor job determining the sub distance, so this now leaves the issue of optimizing the sub distance unaddressed. Some people have run MSO a second time with a "mains + subs" configuration after running Audyssey, but this is somewhat overkill for just a distance adjustment. Dirac users have found that Dirac does a very good job determining the sub distance, so they don't have to mess with it any further. Another option is to do a variant of the manual "sub distance tweak", using the REW RTA with a periodic pink noise source ("pink PN") to measure and observe changes in the combined mains and subs' response in real time. One can adjust the sub distance and observe the response changes in real time due to these changes. REW author John Mulcahy has posted the
REW RTA settings for this scenario. This is one we may have to play by ear. I haven't had to deal with this myself, as I'm a two-channel guy and I only do EQ below 200 Hz - no room correction.
