You already know this but the sound stage depth will be limited by how far your speakers are from the wall. Since you can't move them further away from the front wall I suggest removing the TV and filling the space with a diffuser. This will effectively "move the wall back" a bit. Also, swapping the glass front pictures with something non-reflective will also help. IMO this is the best upgrade you can make at this point. Adding better/more expensive gear will have the benefits compromised by the room not being optimized.
I totally agree with the addition of a pair of subs. Bass traps really help in a two channel system. My soundstage became much cleaner after adding mine. Good luck and cheers.