Have you read up on limp membrane tuned traps? Might be a good solution in your case.

European accommodation tends to be way smaller tan the Northern American Continent's equivalents. I could fit two normal UK houses into my nothing-special home in Nashville. Even the garage was bigger in terms of square footage than my current house! Hence the suggestion of a tuned membrane for his specific mix position problem.

Invite 5 sumo wrestlers to watch you mix, problem solved. Wh0t? :) :p

Definitely worth trying a panel / membrane absorber / resonator on the ceiling. A stiff (plywood?) panel may be suitable. Calculators here: I'm starting to fit out a basement with more floor area but a little less height than your room and I'm expecting to have to do something like that. I'm not tall, so it (and I) should fit. You can do quite a bit of good with only 10cm depth. And some luck.

I'd concentrate on the area above the listening position. A resonant panel is fairly simple and cheap to build. It's just a box that you fix to the ceiling. So it's probably a good idea to knock up a prototype and measure it in situ - stick a contact mic onto it. If you need to tweak it to nail the frequency you're after, it should only involve a bit of light woodwork. And then you can knock up another one to your refined design - or as many as you need. I'm expecting to need at least two panels 60x240 cm. Since you're not after broadband absorption I'd be inclined to damp it quite lightly - maybe put a little loudspeaker wadding in there rather than stuffing it with rockwool. ... checks calculator - a 6mm ply panel (ca. 3kg/m2) might need a depth of up to 23cm for 72Hz Would that be too much for you? Thicker (heavier) panels are less compliant / bendy and probably less effective. An alternative might be what they call "mass loaded vinyl" (back to ivansc suggestion - limp membrane - but on a ceiling?) ... and another thing, sorry :-) How you mount the panel to the "enclosure" is probably a bit of a "gotcha" - if it's rigidly fixed then you're counting on the flex in the panel - if it's on some kind of resilient mount then this is probably less important. You can get permanently flexible sealant/ adhesives for acoustic purposes - I'd try that, with a 12mm ply panel. Less than 12cm depth required. Don't sue me if it doesn't work.

I mix in a room of almost identical dimensions to yours. It is heavily treated with absorbers on walls, in corners and hanging from the ceiling from this one: Their prices are reasonable, but I did find some sloppy work here and there which I had to hide from vision. Still I am not sure if my room is good enough for mixing. But it is what I have. I supplement with reference music and checking with headphones with software correction and room/speaker emulations to compensate.

Use 9 minutes on this one: "Too small rooms produce large acoustic issues. See where your room size and volume fits into our chart and follow Sam Small through his too small room fix."

I have Morphit which is fine too. I have also tried other software emulating everyting from car stereo to tv speakers. However, I generally find they sound more like a recording through a microphone in those places rather than the experience I get when actually listening with my own ears in the same situation.