living room as an Audio Laboratory

You have a great system to start with. Prior to expending dollars, I would take the time to listen to different systems and get an idea of what your sonic preferences are. Ones best system/$$ is that one tuned towards ones own preferences. What you learn will be applicable regardless of room size. For now I would stick with your Marantz integrated and CD player and focus on speakers.

You will learn most by swapping out speakers. Given your current room size and that you have stands I would give a listen to different stand mount speakers that are compatible with your current integrated. You can buy used ones and sell them more easily as shipping costs are reduced. I once owned the paradigm 20s V.4 and liked them very much, however there are so many other good ones in same price bracket offering a different sonic signature. The idea of adding a subwoofer is credible. 

One piece of advice, purchase a copy of Robert Harley's 'The complete Guide to High End Audio', 5th edition. Best $40 you will spend. Can be bought from Music Direct.

I have a very large room in one home (15 x 28 x 11) and a very small one in another (13 x 10 x 8). One can only undo such a small room so much. My best solution is a Netherland product call Zilplex. It is eleven little 1/2" hemispheres that sit on eleven little cradles on your walls. Google that name to find out more.

Frankly, in my small room I could not listen until I added them. They are very unobtrusive.

Yesterday I bought a pair of JSE .6 standmount to rotate with my Silverline Minuets in my bedroom.

Cost was $150 with stands. To me they sound great - outstanding for the money. My girlfriend loves them and sang several songs including all the lyrics to American Pie.

To me, this hobby can be fun and relatively inexpensive if you educate yourself in the forums and keep an eye out. 

I'm sure spending more wisely would result in better sound, but I enjoy going about it this way.