50% is probably doable. May I suggest then that you use
Green Glue and one layer of 5/8 gypsum on all walls and ceiling? If installed per manufacturer instruction this can increase TL up to 10db - which is 50%. and adding a layer of damped OSB to the floor like Ted suggested will help - you may gain 3 -4 db TL there - but people
in the house
will hear the drums.
But this will save you real estate.
Sealing is very very important. Make sure that all cracks, joints, and trim work are all caulked with an acoustic rated caulk. (pretend that you are making a refrigerator room)
But don't put your speakers facing that sloped wall. SBIR - look it up.
As for treatment have a look at some of
my pubs,
GIK Acoustics, and
Real Traps for information and suggestions.
Cheers,
-John