What do you feel is the 'weak link' in your system based on how you use it?
Upgrading the Rega P6 will be a significant upgrade - my initial concern is you won't be able to realize the difference with the home theatre receiver. The change in materials on the table and the better cartridge are very discernable in a revealing system.
I'm not familiar with your specific units; I experienced a significant upgrade when I transitioned from home theatre receiver to an integrated amplifier. Going source first provides future value if you upgrade the pre-amp/amp down the road with equipment more revealing than your HT amp.