I was experimenting 6.0.6.0 and when <copyts>1</copyts> is set in the device profile the subs sync ok and the seek timeline bar works (just found this out now ).
in the beta you sent when <copyts>0</copyts> the subs work but the seek bar is not working , and if I set <copyts>1</copyts> the seek bar works but the subs get out of sync.

for now I reverted to 6.0.6.0 using the <copyts>1</copyts> since both sync and subs are working.