Do you find the power currently lacking? I am all for adding seperate power amps, in fact on my Onkyo 805 I have three outboard stereo amps (one bridged for my center) one for my side surrounds and one for my rear surrounds and the Onkyo 805 now has enough juice to drive just my mains.
The 5308 has a huge amp and by relieving that amp of having to drive other channels then more power is available for what speakers are hooked up, but you have to remember that most of the time your amps are chugging along at just a few watts (3-5 watts 90% of the time) so unless your in a huge room, your blasting the movie and musics to ear deafining levels or your getting distortion out of your speakers during certain scenes during movies than what you have is fine.
If you think you'll get better sound by adding an amp, I can tell you that your money will be better well spent on room acoustics or better speakers and subs than on outboaed amps which will do nothing more than give you a 3db more output than what you are currently running and if your Denon is not giving you any sound issues, then the outboard amp is not money well spent.
