I'm finally back from my vacation and I'm back on this topic.
I discovered my mistake in converting ISO files: I used the parameter [-2] (stereo) instead of [-m] (multichannel).
By modifying the parameters I was able to get DSD 5.1 files from my ISO archives.
So I used 5.1 DSD files on MC setting bitstreaming on DSD and getting 5.1 sound. The MC now plays the 6 channels in this order: 1 FL, 2 FR, 3 center, 4 bass, 5 SL, 6 SR.
Considering I have a 4 channel discrete amplifier and 4 speakers, I cancel the center channel and I add the subwoofer to channel 5 SL using the Okto DAC setup. A blasphemy for purists, but I did it.
I also noticed that the subwoofer channel has a very very low volume, down to -150db. I do not know why.
This way MC sounds very good, but considering I use to play files of various formats I am forced to change the MC setting for different files. In any case with the DSD 2.0 file, most, MC only plays 2 channels and 2 loudspeakers are mute.
So I finally set the MC to "Bitstreaming NONE" and 4 channel output, with 4 channel up / down mix.
Using this configuration I am now able to play all my files, DSD 5.1, 5.0, 4.0, 3.1 and FLAC 2.0, 5.1 achieving a quadraphonic / surround result (real for multichannel and upmixed for stereo files) without continuing to change the MC setting for various files.
Yes, I know I don't play direct DSD and lose something, but it's a very easy way.
Thank you very much.
Have a nice WE
Marco