Dear Mohit,
It depends on the amplifier, I actually find that adding a good two channel preamp usually with tubes, or a high end integrated amplifier gives the best sound for a combined music and theater setup.
I usually run a tube preamp/solid state amplifier and use the AVR to drive the center and surrounds.
The Arcam is an excellent receiver and is one of the best of the breed, however, I have found that there are very few high end surround receivers that can beat the best dedicated two channel gear and that includes a high end integrated amplifier.
You may not notice a world of improvement by just substituting an external amplifier for the ARCAM's built in one. You would get most likely an improvement in dynamics and perhaps a bit tighter bass.
One thing I would do after looking at your gear list would be to first try a better dac. The AVR 600 is now a few years old, and there have been some major improvements in digital processing.
I know a host of DAC's that will improve on the performance of the AVR 600, based on advancements made in digital technology recently.
I am guessing that the sound stage and musicality of the system is missing a bit.
Also good power conditioning and power cables can help the system come to life.