I also traded my 650 for a M6 and I have the RPi GT full (sections 1, 2 & 3) and the sound is great. A little loud but I love it. you can also get the street version which is about 20% quieter.
Plus factor, you will add HP power to your bottom line.
90% of M6 owners that I know have the RPi for the performance and sound,
The only wat to judge is to listen to all in person as the videos really does not help, it sounds good but in person it will sound totally different.