My take on this.
- 671 assumes large speakers to be the ones with woofer size more than 16 cms i.e 6.2 inch. This includes the hard edges of the speakers and not only the woofer size. I'm not sure when other companies mention woofer size, if they include the edges as well.
- Typically when the speakers are marked as large, more low frequencies are sent to the all the speakers. So the sub is fed (through YPAO adjustement) with as low frequency as possible. In your case 40hz.
- Most 'decent bookshelves' are capable of producing till 60 hz (exceptions are always there). In your case the rears are at 65hz before roll off.
- Since your sub is marked at 40hz, therefore all other frequencies would be sent to other speakers, including the rear bookshelves. Note they cannot reproduce frequencies lower than 65hz without rolloff. So you will find a dip in the band 40-65hz in the rear, if at all the sound travels to the rears at those frequencies. You can adjust for this with the speaker level equilizers by increasing the sound for the frequency band, but then again it would be a strain on the bookshelves.
- Typically all sorts of sound/vocal are taken care off till 60 hz, baring some. So if you manually overide the YPAO (after running the YPAO) and set the speakers to large, but mark the cross over frequency to 60-65 hz, then the 'complete' band of sound would be handled by all the speakers, where as the sub will take care of the very low frequencies and not strain the speakers. This will also ensure the sound of jet fighter stays the same when it is travelling from front to rear channels and not become lean as it moves to the rears. Assuming your bookshelves can actually play 65hz without roll off.
- Adjusting the reference db is a matter of what you are listening to. The YPAO will by default set 'centre' speaker a bit high for better speach (40-75% of a movie, depending on genre). If you play around with DSP settings you will notice it adjusts all these parametres based on the genre of movie you are viewing. For e.x set to high dynamic ratio across speakers for 'Sci-Fic' movies, dominant speach from centre for 'Drama' etc.
- There is a school of thought who consider the sub set to 120hz is good for rest of the speakers. One way it is that all other speakers are not strained, also there is no phase issue. But I feel till 60-80 hz typically the sound that is created is more follows axis, kick drum, low male voice etc and therefore they should be produced by respective speakers rather than one corner of the room. Frequencies lower than these create 'ambience' and they do not have to follow a particular axis. But having said that its very subjective.
- Try playing around with different setting and internet and it should be fairly easy.
