Connect 3 sets of phono leads from 575 multichannel connectors to 2400 multichannel inputs. You can select the area of the SACD (hybrid) disc that you want to listen to from 575 menu. Then select multichannel input from 2400 unit/remote to listen. You may want/have to set the speaker levels differently for multichannel input as the 575 doesn't do bass management (or at least not very well if it does) - you can change the multichannel levels independantly of your normal settings using the level button on the 2400 remote while in multichannel input mode (assuming that it is the same process as the 1400).
Well it does remember the settings on the 1400, but I think you can only have 2 settings - 1 for multichannel input and 1 for the rest. I ended up setting my levels on normal input by setting the subwoofer level to -10 and then adjusting the gain on the subwoofer to get level balanced to my other speakers (ie 75db) and then setting the subwoofer level on multichannel input to +10 - at least now I get a fairly decent bass from SACD/DVD Audio.
Yes I agree the manual/menu system is a bit complicated but it does get a bit clearer after reading it a few dozen times