Complex sentence: In 2015 his rediscovery was sparked by the release of the first two volumes of his live performances, leading some modern-day commentators to declare him one of the greatest pianists of all time.
Split into simpler sentences: In 2015 his rediscovery was sparked by the release of the first two volumes of his live performances. Based on these some modern-day commentators have declared him one of the greatest pianists of all time.