Complex sentence: The Estonia pianos manufactured during the existence of the USSR were surprisingly strong and could demonstrate excellent stability, fast mechanic and grand singing tone, they are very rare to find nowadays and are almost an extinct brand.
Split into simpler sentences: The Estonia pianos manufactured during the existence of the USSR were surprisingly strong and could demonstrate excellent stability, fast mechanic and grand singing tone. Pianos from that period are very rare to find nowadays, and are almost an extinct brand.