Final sound is not definitely set in place, there are still some possible tweaks happening with the amplifier circuit. But what I would personally say(so according to my ears), M8 sound signature is closer to the M6 and Shanling's house sound, compared to little bit more neutral leaning M6 Pro.
Difference between devices is mainly in sound quality. M8 also has better screen(5 inch 1080p) and we updated it to Qualcomm CSR8675 for BT, for improved both-way Bluetooth functions). And of course the new mechanism of headphone sockets.
Otherwise system is similar, based on same hardware, so you can expect the complete feature packages of M6/M6 Pro(USB DAC, digital transports, both way bluetooth, open Android,etc, etc).
We hope to have M8 on internationals markets during fall.