Thanks for your advice guys, i really appreciate it!
Francis, i did probe the room with sinewaves (using files from Ethan Winer's site) and an SPL meter. However, i played both speakers simultaneously. I reckeoned, that would resemble the normal situation. So it's better to do it with one channel at a time?
Learned something again
I might come up with a plan using both panel an slat absorbers. Did some reading in the meantime and lost some fear of the latter haha. Like you said Glenn, with the slanted front and a some mufflin' against the back of the slats, they will have a broader Q and become more 'forgiving'.
The strongest and lowest peak (60Hz) is mainly at the front-floor corner.
So i may not need absorption covering the entire height.
At least at the rightside (it has a few narrow windows), i hope that i can keep some daylight coming in by utilising one of the side absorbers only half height. It seems that the offending resonances are present at lower regions anyway.
I realise that i should not forget first reflections this way. But if i keep those smaller tubes at the side in place, that should be covered. If not, then the side absorbers should have sufficient height and will be dressed with soft material or maybe a diffusor. I assume that you can cover slats with a soft surface as long as the slots remain unblocked.
As Glenn mentiond earlier, it's a good idea to look (measure) right above the listening position. I already know that some peaks exist there as well. Depending on the broadness (or not) of peaks, i'll determine what type of absorber will prove best.
No cloud yet, just a panel absorber above the speakers, almost against the front wall (plus one at the rear end).
As to the top-floor positioning, there's not much i can change about that. There's a comfortable working height, which dictates tweeter height and the ceiling is fixed obviously...
Since this is all very important and interesting stuff, i ordered a book called 'master handbook of acoustics'. I think all studio techs should have a basic understanding of this. At least i want to learn more.
Thanks again for your time.
(And Glenn, congratulations!)
Cheers,
Peter