Good info so far.
I'll be emphasizing a bit of what has been mentioned so far, but will add a few things to the discussion that I hope you will find useful.
Build your own bass traps and install them in your room as shown below:
Here is an Over Head image of the same thing:
Make your own bass traps. It's easy, and this will be the single best thing you can ever do to create a great sounding recording, mixing environment. It will have the single greatest impact on how your music sounds in your car, boom box or any other medium on which you might play your music. No amp, preamp, converter or anything else can make this big of a difference in how you hear your music. I guarantee it.
Build: When building your own bass traps, use rock wool, OC703, or another 3lb pcf acoustic insulation. Use a fabric you like and that fits your style. Make sure when you hold the raw fabric up to your lips and blow, that it offers a modest amount of resistance (at least). This will ensure that insulation fibers will not creep out of your panel and into your room.
Here is a DIY Bass Trap tutorial that utilizes materials you can find locally. This is based on a design created by the folks at
http://forum.StudioTips.com.
Here is some additional, useful information to consider when setting your space:
4 Rules for Broadband Acoustical Treatment for Critical Listening Environments
1. Lay your system and listening position out symmetrically Acoustically render the left half of the room as an exact mirror image of the right half of the room, especially the part of the room and system components in front of the listener asymmetries behind the listener are less of a problem than asymmetries in front of the listener.
2. Use broadband absorbers [minimum 4 / 100 mm thick] so that your treatments work on all frequencies [lows, mids, and highs] in a linear fashion [evenly / smoothly].
3. Smooth out the non-linearity of very low frequency resonances which exists in all small rooms by installing 4 thick corner mounted treatments. As a standard guide for optimal treatment, add a pair of vertically stacked 4 thick corner panels in each of 4 room corners [total = 8 corner mounted bass traps]. If vertical corners are not available, then use any available horizontal corners [ceiling/wall or floor/wall corners]. If you cannot install 8 corner panels of 4 thickness, then use thicker panels in the corners you do have available.
4. Treat Early Reflection Points with additional broadband absorbers. As a standard guide for optimal treatment, hang a pair of panels on the front wall, 1 panel each on the side and rear walls, and 3 panels on the ceiling each located so as to mask the first reflection points [total = 8 reflection point panels].
I hope this information is useful to any interested in a great sounding room, mix, etc. It truly is the best upgrade you can make to your space, and may be one of the least expensive when you consider DIY options.
Joel DuBay
www.readyacoustics.com
www.basstraps.net