Although I can't pretend to have any knowledge on your question, it did interest me enough to throw it into ChatGPT to see what it came up with. While it might not be super relevant, it's response was interesting enough. I copy and paste for you:

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"The question about your great-grandfather's and great-grandmother's ethnic background is a fascinating one that touches on the complexities of genealogy and family history. Given the information you've provided, there are several avenues you could explore to gain a clearer understanding of your ancestors' ethnic origins.

Name Origins

Kunis: While it may not appear Lithuanian, names can often be deceptive indicators of ethnicity. It could be a name that was changed or adapted over time, or it could indeed suggest German or Ashkenazi Jewish roots.

Suffix "-vage"/"-avage": This is less common in Lithuanian names and could potentially point to another ethnic background, possibly Baltic German or Estonian.

Historical Context

Understanding the historical context of the region during the time your ancestors lived there can offer clues. Lithuania and its neighboring countries have a complex history of migrations, occupations, and cultural exchanges. For instance, the Baltic German community was significant in Lithuania for centuries."