How do you enhance a bad listening room's accoustics without breaking the bank? thoughts?

Heavy draw curtains floor to ceiling. You can open and close for the amount of dampening you want. I've always used heavy rugs placed in front of the seated position. You want to absorb the sound more than diffuse it...

Decoupling is a good way to reduce bass issues, for you and the neighbor. Mains and subs..

For the ceiling a super cheap way is fishnet on the ceiling, then cellophane, then another fishnet to hold the cellophane in place. You can dress it up pretty easy. Put it up or take it down for cleaning in 15 minutes with just a little help.. I had about 30 peacock feathers in mine. STOPED the HF boil I was getting, pure room first point reflection issues..

I use small ribbons and planars.. you have to develop a plan with that type of driver. :-) 

GIK good stuff.. 

Regards

Get 6 Stillpoint Aperture panels for The best sound  ASAP.Hope the bank will be ok.

With acoustics it is best to identify a problem before solving it, either by listening or measurement. How do you know if you have a reverberation or reflection problem or bass problem that will improve by adding this or that? Cork will not help. Even mid-high frequency absorption will not stop bass transmission from bother neighbors. Sound proofing and acoustics are a bit different. The good news is mid to hi frequency absorption and diffusion can very had for small amount of money, especially with DIY.  I would learn by way of https://www.amazon.com/Master-Handbook-Acoustics-Alton-Everest/dp/0071603328 before wasting time and money and creating garbage. That book will provide general usable knowledge as well as practical suggestions. I have been there myself. 

Just placed an order from GIK.  45 days out for my hybrid diffusion/absorption/trap panels.  My wife approved them as they are quite attractive.