How To Proceed With Room Treatments

   I live in a house that is more than 100 years old. My listening room is approx. 16' x 20' in size. The floor is carpeted. There are furniture pieces. In the days this house was built the walls are of plaster over wooden lath rather than conventional drywall.
  My system (for music only) consists of a pair of stand mounts and two 10" subs. I have experimented with moving the speakers to various locations, moving furniture, and have now found placements that sounds best to me. However, in the quest to improve the sound as much as possible, I am interested in the possibility of adding some type of room treatment. While there are many options such as wall panels, corner bass traps, etc. is there any sort of experiments that can lead to a final room treatment that can optimize results before buying the rather expensive panels for that purpose?
I realize this is a very subjective topic but am curious if any have added some sort of temporary material to their rooms to determine how to proceed with a permanent solution.