Using External Speakers with a Lumina Frame or others...

[daverob]

So are you saying above, that the signal that goes through R5, R6 is the "not amplified" signal, so to say the pre-out signal?

That is correct.

Than wouldn't it be a proper solution to replace r5, r6 with a stereo potentiometer and leave the volume switch in the "loud" position (in the r5,r6 only position)?

You could do it by fitting a stereo potentiometer across r5,r6, but you must not remove r5 or r6 as they set the maximum volume for the amp, and would cause the amps inputs to be overloaded if the potentiometer or it's connections were damaged, potentially causing damage to the amp/speakers. I'd recommend using a 10k stereo pot across these resistors, the maximum volume level would be slightly reduced, but you should get a good range of adjustment. A 5k (or 2k) stereo pot would also work and give a finer adjustment at the expense of lower maximum volume.

so with r7, r8 in parallel it sould be loudest, r9, r10 medium and only r5, r6 sould be quiet...

This is incorrect, these resistors are used in a potential divider to attenuate the signal, so smaller resistance values will result in lower volume levels (ie what I originally wrote is correct!).