What a difference curtains make...WOW

We just moved into a new townhouse with hardwood floors and a large view window. We placed an area rug in the middle of room. Here, I will not be able to have a subwoofer because people live in an apartment below our main floor (we have 3 storys, but an apartment is wedged between our lower level and main level). Since moving, my Polk Rti28's, I thought sounded tinny and bare. Our new, thick curtains arrived and I installed. Today, I turn on the stereo and WOW!! my speakers seemed like a whole new set. Bass was good, and everything sounded uncolored and natural. Wall/floor treatments do really work.
The more room treatments (even if they are thick curtains) the more you hear your speakers and less of the room.
Most often the performance of a system in a given space isn't optimised due to room conditions. The performance of a system will be elevated to a much higher level in a good room coupled with proper setup, placement and acoustic treatments. Since most systems are often located in the living or lounge area and not in a dedicated room, one has to live with compromises(WAF) and accept any effects that the existing space may bring to the system.

As you have found out, even a minimal change in the addition of thick curtains is able to alter the sound to a significant degree. You can thus imagine how the system would sound like if more thought is given to improving room acoustics.
Your neighbors concur!
I was given some quilts that my Grandmother made during her life. I had them professionally cleaned and hung on the walls throughout the house....and WOW! One of my rooms that sounded terrible before, now is my favorite rooms to listen to music. Thank you Grandma for the room treatment, and the memories.
I am now taking time to listen everyday now. I believe I finally have good speaker placement as well. I actually think my system sounds better now than it did before with a subwoofer. I read somewhere (maybe TAS or Stereophile) that the weakest link in 99% of home audio systems is the listening environment.
Now try putting some GIK corner traps (or 244 panels) in the corners of your room to absorb the boomy low end and report back.
Any suggestions on curtain material?
Be careful not to overdo room treatments, as they can kill the life out of your speakers. Treating the room is a necessity, but you can easily hear the point at which the sound is just dead.
It also depends on your speaker design. Some designs doesn't require heavy room treatments but more detrimental to the speaker performance when used, while others require extensive room treatments and environment placement. It just depends on the design and type of equipment used. curtains or heavy drapes can absorb too much sound waves on some systems which makes the system dull, while others helps tame brightness or over reflection. Some sound engineers used to warn against placing loudspeaker near draped locations in past findings. If your loudspeaker requires hard floors and solid walls to perform it's optimum, treating the room with drapes or curtains may not be ideal. Some bipolar or dipolar designs are heavily affected by such set up.