Hi gigirex, I once have the same requirements as you and I call some customer service center for help, and most of them tell me that any conversion need to recode, so the quality loss is unavoidable anyway, you should also remember that if the output bit rate is smaller than your input bit rate, the quality would be bad. So a suitable bit rate is a must while converting. And I used ojosoft MKV to MP4 converter to settle my problem at last. You can freely set your bit rate, frame size and frequency range by an edit button. Maybe it is suitable for you, too.
http://www.ojosoft.com/how-to-conver...converter.html