But that's a bug in the speaker configuration function. The solution is to fix the specific problem (i.e., allow the centre to be set as 'none').

True, but if steering is available at all, what earthly reason should there be to not permit it in all modes? While likely necessary for PLII and PLIIx decoding gone awry (i.e. from ordinary stereo sources), I can see it useful for discrete sources for two reasons: (a) to accomodate for badly encoded material, with too much folded into the center (a PLII to DD transfer, perhaps, if unlikely?); (b) (which is more likely), a customer using a ribbon centre channel speaker, like the BG Radia 220i. Such speakers have a cylindrical dispersion pattern, and one would think that a centre speaker would have the ribbon driver mounted at 90 degrees to offer a dispersion pattern rotated with respect to its orientation (i.e. horizontal in the room). In fact, this is not done: a vertical (non-rotated with regard to the speaker) dispersion pattern is used for the benefit of listners straight ahead of centre. This avoids listeners to the side placing the center speaker off to the opposite side. But, the signal drops off rapidly to the sides (so instead of misplaced center, you get almost no center). Blending some of the center signal to left and right avoids this problem.

Some go so far as to argue that, with ribbon L+R mains, one should not have a centre channel speaker with high frequency response, or any at all. I can vouch for this: I'm currently running a 2.1 setup with Radia 520i mains and a sub, with a stereo downmix sent to the mains, and centre is dead centre even up to 30 degrees off-axis. One would think that centre would move closer to the nearer speaker in this case, and it does, of course, but the effect is attenuated by the fact that ribbon drivers' output falls off at 3 dB as one doubles the distance, not 6 dB, if one would have a spherical dispersion pattern (i.e. with a conventional speaker).

In lieu of a fix for the bug, it is frustrating to not be able to hack a fix by using centre channel steering in all modes.

PLIIx can be applied to 2-channel and 5.1/6.1-channel material.

Yes, but it always struck me as silly to use PLIIx to go from 5.1 to 7.1.

It was when Pro Logic first came out, but since 1988 that decoding has been done in the digital domain.

Granted.

I just always thought of matrixed (as opposed to discrete) surround modes as "analog" because they could be recovered/processed from two-channel analog matrixed mixes (i.e. the stereo audio connections from a DVD player). That was never done for discrete multi-channel audio carried over analog lines, because it was already decoded. I suppose the closest would be external bass management/processing for signals from 5.1 analog outputs from a DVD player, but even here, the decoding has already been done.

That's 17 years ago.

You haven't been around very long, have you? At my age, 17 years ago feels like "yesterday".