Question: Sound Treatment behind the Listener?

I have found the Acoustic Insider channel on YouTube to be a good source of information around treating your room.  He mostly discusses treatment for small home studios, but the theory still applies to a listening room.  

As far as I can tell, he's not selling anything.  He does reference a document you can download, and I have no idea if he charges for it or if that is a gateway to purchase other stuff, but I've never looked into that.

One of the things that I consistently hear in my research on acoustic treatments is that you can never have too much bass absorption. The problem is that you need really thick panels to get into the lower frequencies. If you use thinner panels, they will just absorb the higher frequencies (which has its place in making a room less reverberant or maybe comb filtering) but won’t help you with bass nodes or standing waves.

Even if you get thick absorption panels. they still absorb the top end so the theory is that you should just have the thickest absorption panels you can so that you get as much bass absorption as possible.

So if you decide not to get the Alpha 6s, try to make sure you make your panels as thick as you can. The Alpha 6s are six inches in depth with a scatter plate so the theory is that they would absorb the lower frequencies while providing diffusion for the upper frequencies.

And, in my experience, it’s so hard to tell. I mean, while I get controlling excessive reverb and reflections makes total sense on paper, there are so many options and so many people (including me!) who just repeat what they’ve heard without direct scientific experience it’s frustrating. But you have time. And it never hurts to try the free or cheap route and work your way up to more expensive things.

Good luck on this journey!

I honestly have no idea.  The Podiums maybe help with vibrations, but I don't think it would help with bass?  But I am not an expert.

If you think the bass sounds good from where you sit, I would just not worry about it.  In the end, do you enjoy listening to your system?  That's all that matters.  

You may not need any (or you may need a lot?) of extra bass absorption.  It really has to do with your room geometry, where the speakers are placed, and where you sit.  The guy from Acoustic Insider always suggested figuring out where the best place to put your speakers and the best place to sit is the first step in "treating" your room and that is free.  

If you are sitting in a null, you won't hear much bass and if you are sitting in a node, the bass may sound too loud or weird, so you'd move your seat up or back to see if that helps.

Your original questions was about if you need to do anything behind your listening position.  The challenge with your CD wall is that it could be very reflective and late reflections from that back wall could cause some issues.  If it sounds good to you right now, again I wouldn't worry.  If you want to experiment, try something behind your listening area that is not super expensive (couch cushions?  Thick blankets?  I don't know....) and, if you feel it helps, then consider purchasing or making something more permanent.   

I have a window behind my seat (about 5 feet behind me) and I have two 4" panels on stands that I just put in front of the windows.  To me, it helped a LOT.  It was probably one of two room treatment things that made me go "yeah, I can tell a difference".  But that was when I had no other treatments up at all.  You already have a room that is treated so your experience may be different.