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

@ pooch2, what you are making a reference to is ' Sound Proofing ' and not Acoustic Treatment ... "  reduce the noise coming from my stereo ".
Low End will ' seep ' into your neighbors space unless extreme steps were taken during the original construction.
The best option you have - if you do not want to offend your neighbors is to keep the volume low ... even so Bass will be noticeable to an extent. 

Materials have some combination of the four basic acoustical properties: Absorption, Diffusion, Reflection, and Transmission. Materials vary in their performance of those properties with respect to frequency. Cork is absorptive only at relatively high frequencies but is very damped with respect to transmission. A flat cork wall does little if anything to help reflection and diffusion.

Adding a floating layer of sheetrock hung by Z-clips to the adjoining walls, and edge sealed with acoustic caulking will reduce noise transmission between units, and if possible, is strongly recommended.

Within the room, acoustical panels help control unwanted reflections via absorption, as will a heavy carpet. Diffusers, as the name implies, help scatter sound without absorbing it. Bass traps help reduce low-frequency standing waves. 

Two to four sidewall acoustical panels, 2' X 4' and a pair of bass traps would probably significantly improve your overall sound . My own preference is Acoustimac for choice, cost and customer service. 

To help with room acoustics it’s typical to get absorption at the primary reflection points and, for bass, in the corners and to some degree diffusion but in smaller rooms absorption tends to be a bigger issue. The trick is getting the right amount of absorption without killing the space. These products tend to incorporate rockwool and fiberglass of varying densities with denser products absorbing lower frequencies. Foam panels are often used but just aren’t as effective as rockwool and fiberglass as thin foam only tends to absorb the higher frequencies.

Now if you are looking to reduce sound escaping into your neighbors condo that’s a different treatment. There you want mass, multi layers of gypsum board (drywall) or better yet chase walls built with an air space, then wood studs with batt insulation then multi layers of gypsum board. Better yet again, concrete block. Eliminating sound from entering another space is much more difficult than acoustically treating the listening space itself. Bass and the long wavelengths associated with those signals are the enemy for sound control.

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