If there is no centre speaker, then the centre channel content is split evenly between the front left & right channels. This is done automatically as a function of setting the speaker configuration, so there's no need to manually control centre channel steering.

Except I can't seam to tell the 990 that there is no centre speaker!

BTW, what do you mean by digital and analogue matrix modes?

Dolby PLII and PLIIx are matrixed into two channels, no? Historically Dolby Pro Logic decoding was done in the analog domain, IIRC. So, I tend to think of matrixed multi-chanel audio as "analog", because it can be carried over a conventional stereo analog communication channel.

Dolby Digital and Dolby Digital EX (and other) formats are discrete 5.1 and 6.1 channel formats, often found on digital media. (Naturally, one could also have PLII and PLIIx on two digital media audio channels).

Digital processing is possible for all these formats. It's just that it was not done that way originally for plain old PL.