Thanks Varpa!
I was unable to launch qtractor! I don't know why!? other apps too! it is not the first time that any apps won't start. (AVlinux5.0.2)
Then because Avlinux5.0.3 is there, i decided to install it! Ok now i've got qtractor running... I tried it a little bit, but because of soloing busses (one mute the other) is apparently "not possible" it was too much to manipulate and monitor... Then

i tried with ardourvst (for possible vst) and i added some busses for routing direct all height mixbusses of Mixbus in one bus in ardourvst... and the other bus (in ardourvst) monitor what goes out of channel 5/6 and all that linked (out) to channel 3/4 as desired (because my active monitors are linked to 3/4)...
Ok many routing are available like this and it is possible for me to switch soloing tracks (option unlatched)..
Here are descriptions of Solo and co....:
http://ardour.org/files/manual/sn-tracks-and-busses.htmlIt is very helpfull because you can compare two (or more) mix just with one click!
Maybe you're right for the processor consumption because i don't see how qtractor took but ardour (Mixbus here) took about around 5%!
Then I don't remember if JackMixer can afford the "unlatched soloing" but i think the power consumption is not so high isn't it?
Then i think of an extra-minimal ardour just for this kind of application

If someone has can give me a way to try nonmixer (as package) it would make me pleasure to give you a feedback!
Thanks varpa for your tip!