The mode presets in the Setup / Source screen are simply the defaults the AVM-50 uses each time the AVM-50 detects a new style of incoming audio format from some particular source device when you are listening to that source.
You can manual change to a different form of surround sound processing while listening by pressing the Mode button on the remote and then using the up and down arrows to cycle through the available options. The options available will vary depending on:
1) The type of incoming audio signal the AVM-50 currently detects.
2) Whether or not you have rear speakers configured in addition to surround speakers.
3) Whether or not you have turned on THX post processing -- by using the THX button on the remote and then the up or down arrows to toggle the setting, or by selecting THX on "by default" in the appropriate Setup / Source line.
If you would prefer that the AVM-50 *NOT* have a default surround proccessing mode for any given source device or for any specific type of incoming audio format from a source, simply select "Last Used" in the appropriate Setup / Source lines and it will use whatever you last used for that type of incoming audio.
Here's an example:
I prefer THX post processing turned on while watching movies and off while watching TV. This is true whether or not the movie or TV content is 2 channel or 5.1 I also don't like THX added to DTS content. These, by the way, are all personal preferences and your preferences may be different.
So for my DVD player input in my D2 I have 2 channel defaulted (in Setup / Source for DVD) to PLII-x with THX, and 5.1 Dolby defaulted to DD with THX, and 5.1 DTS defaulted to DTS without THX.
Meanwhile for my cable TV input in my D2 I have 2 channel defaulted to Anthem Logic Cinema (no THX), 5.1 Dolby defaulted to DD (no THX), and 5.1 DTS -- which is unlikely to ever appear on a TV program, but it doesn't hurt to set it up -- defaulted to DTS (no THX).
Now if I happen to be watching a DVD recording of a TV series, the default will have THX on -- which I *DON'T* want -- but I can easily turn it off for that viewing using the THX button and then the up or down arrrow on the remote. The default I have set up stays remembered, and works for me for most DVDs I watch, since most of them happen to be movies.
And if I happen to be watching a TV broadcast of a movie, THX will be off by default -- but again I can easily turn it on for that viewing.
The various processing modes you have available are described in the user manual. Anthem Logic Music (AL-Music), for example, is a non-aggressive surround sound mode for listening to music developed by Anthem. It is designed to give a little bit of AVM-50 generated surround (as compared to "Stereo" which is straight 2 channel without any generated surround, or "PLII-X" which produces a more aggressive form of generated surround sound), and it specifically DOES NOT USE the center speaker so as to keep from screwing up the "sound stage" for the main stereo speakers. [NOTE: Some surround modes have options you can adjust. For example you could, if you wish, adjust PLII-x to not use the center speaker as well and the AVM-50 will remember that setting for that specific mode on that specific input.]
So if you are listening to an incoming 2-channel audio signal, and AL-Music is your default setting -- or if you happen to have selected it manually -- then the center speaker will be silent. Just use the Mode button and the up and down arrow buttons to try different options. If you like one in particular, then go to the Setup / Source screen and set it as the default for that style of incoming audio signal from that particular source device.
The AVM-50 comes with factory preset values for these defaults which are pretty logical. For example the presets for the "CD" input are geared towards music listening and for the "DVD" input are geared towards movie listening. But you are free to change them around as best suits your particular set of sources and the way you use them.
DVD players often default to 2 channel output until you tell them otherwise. You may have to do this in 2 different places.
First in the Setup inside your DVD player, you need to tell it that it is OK to send the undecoded Dolby Digital or DTS "Bitstream" as digital audio output if the DVD disc happens to offer it. The AVM-50 will decode the Bitstream and play it.
Next, when viewing a given DVD disc, you may have to go into that disc's own menus and tell it you want to listen to the multi-channel sound track instead of the stereo sound track that happens to also be on the disc. Many movie discs default to 5.1 sound already, but some require you to ask for it when playing the disc.