I haven't heard the Lina DAC, but he Apex upgrade to the Bartok is significant in terms of the improvement to sound quality - it makes the DAC more musical and less mechanical/digital sounding.
DCS Bartok APEX or LINA and master clock
I have a DCS bartok that I can get upgraded to APEX or I can sell the Bartok and purchase a DCS Lina and Master clock. The cost would be about the same any thoughts on which would be better.
I have a pre amp already so I don't need the Bartok preamp functionality.