What to upgrade next?

Upgrade or tweak? Are you using a line conditioner, if not you should. You might want to consider a Furutech Rodium outlet, I put one in and heard a 20% improvement right away.
Are your cables laying on the floor? Get them up off the floor for a quick tweak.
My 2 cents worth.
Dave

Your speaker cables look like the next step, as everyone says. I like my Ensemble Voiceflux cables for the money, but you can pay more and get better. I agree power conditioning needs a look if you haven't done that already, and also good power cords, especially on the CDP and preamp.

The biggest improvement in your system will come from upgrading your speakers. The cables may add some sound changes but better speakers will open up a new world of sound. Not that I think yours are bad, but look up the line and you will get better bass, mids and high end which to me is the biggest improvement one can obtain.

Happy Listening.

I think you're getting a lot of misleading advice. There is absolutely no reason to replace your cables, nor add fancy power cords or line conditioners. You already have a great system and assuming you've set it up properly, it's sound will not be readily improved without wholesale changes. Changing cables and such may slightly change the sound, but it's unlikely to really improve anything. I have three suggestions. First, consider some professionally designed acoustical room treatments. Dollar for dollar it's the most cost effective money spent. Second, look at getting a high quality rack and speaker stands (Sistrum, Mapleshade, Solid Steel, etc.). Third, replace your CD player with either the Linn Unidisk or Teac/Esoteric DV50. Both players are universal machines that will playback any format which will allow you to run hi-rez SACD and DVD-A digital through your system.

I think Onhwy61 makes a great point.

You seem happy with the sound. And, if you are interested in upgrading, I would ask what area(s) does it lie? From there, we can offer suggestions in terms of pushing your system in the desired direction. It seems as if your source, preamp/power amp, and speakers are on the smooth and warm side, while the cable is on the detailed and fast side. May just balance extremely well, and not need upgrading at all.

I can't argue with addressing the room, it's often the most overlooked link in the chain...