I'd say first better improvements of significance, regarding equipement upgrades are going to be making sure you have ALL MATCHING SPEAKERS in the system, THAT ARE PROPERLY SET UP FOR BEST SOUND! (Maximizing).
HOWEVER, two most effective aspects you should address, for MAXIMIZING your system, as is, is making sure your speakers are setup for proper flat response, as well as the bass management proper setup. (check you're crossing over at 80hz, sub and speaker both coupling to the room/seats well at crossover, as well) You can easily measure each speaker's response with testone discs from numerous companies, one speaker at a time (critical). With all the speakers and seats placed for flat response (again, most NO ONE ever does this - they stick the speakers and/or seats where they think they should go, like some "golden triangle", er whatever.), you are allowing your self maximum dynamic range, accurate response and sound reprodtion (considering proper toe-in, aim, and acoustics).
Also, ESPECIALLY WITH A RECEIVER (yes, even the 5600), you have LIMITED POWER OUTPUT OR CURRENT, and thus need to cross over passive speakers at 80hz, and let the powered sub handle the demanding bass info.
I cannot tell you how important both these aspects alone are for maximum dynamic range, impact, and accuracy of sound!
So, if you want to MAXIMIZE, start there. You're system will thank you for it, and you will too.
As for the gear, even with the 5600, adding an outboard amp will easily double the sonic capabilities (if good amp), as the receivers amps in that receiver are not so great on their own! I know, I sold that receiver, and did "AV receiver-wars" with that piece against the old Yamaha RXV3090.
Back when both of them were the big receivers in 97, I played with these receivers pretty extensively, driving both small and large speakers "full range", playing a variety of music and movie material through the systems. EASILIY, infact not even close, was adding an outboard amp to the receivers FAR FAR SUPERIOR SONICALLY, over the amps that are built into those receivers! Basically, that's where the greatest compromise in sound quality is off the bat. Add an amp, and the dynamics just come alive, and the sound get's way richer, the soundstage bigger, and just plain better sonics all around. (The amps we used were 125w/ch Denon POA8200/8300 THX amps).
Basically, I wouldn't use a receiver's amps to drive a system, except on budget systems, with ultra high sensitivity horn speakers, active-woofer designs, etc. Really, most passive speakers need a better amp to make them sound good.
Other than that, make sure ALL YOUR FRONT SPEAKERS are matched! The rears should be too, but not quite as critical.
Good luck
to drive the speakers
