I think the minimum thickness to dig to or slightly below 50hz would be about 17 to 20 cm. Then you would need less in the way of thickness for initial reflection points. That ceiling would freak me out. I hope that you have a window for a view outside the room and for ventilation as it sure will get warm in there in your cozy room with nowhere for the heat to go. Oh, I am talking about rock wool, by the way, something you can buy from a do-it-yourself store.

 

I saw this, seems like a great deal on Bassotect absorbers, 10 cm thick which would work well for your needs.
http://www.ebay.de/itm/8x-Schall-Da...lles-Hellgr-/111631105385?hash=item19fdbb7d69

 

I'll just post my usual advice that i boringly trot out to anyone who asks this kind of question.

Go directly to http://www.soundonsound.com/forum/postlist.php?Cat=&Board=DESIGN

Theres enough material there to read and enough good people who do that sort of stuff for a living , to get all your questions answered.

Make sure you read some of the forum posts first as you may find the answers you need straight away.

 

i´m not shure if i understand correctly.
You room would be seen by clients?

Of course diy room treatment can yield great results, if one takes enough time with it (planning and calculations)
but your room size would suffer from mass traps, and maybe won´t look as slick
maybe a combination of diy and a room kitt
there are very afordable treatment options like a primaccoustic London 8 for as low as ca. 250€
this with for instance heimholz resonators for specific baass problems
or diy corner bass traps

of the shelf stuff
vicoustics looks amzing imho
primacoustics room kits are great (have yielded me great results) and look good
depending where you live there are plenty of resources out here

diy
caruso isobond is the best price to performance option (and looks, non toxic)
second rockwool and pressed fibreglass (best performance (can have health issuses if handled incorrectly)
if you dont get an amzingly cheap price on basotec i would stay away from it (too dense to have significant results below 150hz)
soundonsound forum is great
also some hifi forums
this one here is cool too ;)
you need time and have to be vigilant about numbers and calculations

same with speakers
you have to check them out in your room!!!!
buy and send them back till happy

hints
around 500
jbl, yamaha

around 1k
+ adam, genelec, dynaudio

around 1.5k
+ k&h, pmc

around 2k and up
+ Munro sonic and many more boutique solutions

hope it helps and of course this is just my opinion
bonus video ;)

 

Isn't both monitors & headphones the best after all?
I mean why use one of them, when you can switch and compare audio?
right?

 

then take it easy mate ;)
early reflections + bass control should be first on your list (because bass control is = reverberation time)
in the video its the "time domain" he keps mentioning

dont over do it with dampening at first
for instance easy if you have carpet

Keep your stereo, wait with the Headphones (if there are no neighbours complaining already)
i keept my old kitchen stereo ;)

and lastly are you sure about the room hight? normaly door frames are about 2m high?!

 

Sorry, u right 2.30M doors 2m :D...

This is how it looks

http://keepimg.com/image/MO
http://keepimg.com/image/MV
http://keepimg.com/image/Mw
http://keepimg.com/image/Mc

http://keepimg.com/image/M3