Originally Posted by khoyme
... so I will use the full 7.1.4 (and I have a second smaller Paradigm sub lying around that I might mix in for 7.2.4)
There are a couple of caveats on using the second sub that is different. The wider the difference between the subs, trickier to set it up but with careful setup you can get better sound than not using the second at all. It is easier for people who use their ears than just auto-pilot with Audyssey because of its caveats.
The best position to use for dissimilar subs are the diagonal front and back (right side in front and left side in back or vice versa).
Audyssey will individually correct for delay and phase differences in the initial single speaker test, so level setting is not usually an issue and works fine to remove those problems. However, it does equalization with both together and here is where large differences between the two in peaks and dips and roll-offs can throw it off. On the other hand, if you equalize with just one sub then just plugging in the other one later won't work.
A trick that works often is to do the initial level setting with both on and then when it gets to the listening position tests, do three different calibrations. (1) With both on (2) With first sub on and the second one's volume turned down half way from its current position (3) the opposite of 1
Once (2) and (3) are done, use your ears to slowly bring back the volume up of the lowered sub to find a position that works for you. This way a steeper roll-off of the second will not compromise the equalization of the first but any unnatural peaks or nulls because of the second or its position will be somewhat compensated.
With decent subs, the above works fine to find a position and setting that you will appreciate better than just one or avoid spending good money to match the better sub.
No guarantees but different subs are not a "definite no-no". This is not an exact science.