Dec 2 (Reuters) - Australia's Bigtincan Holdings said on Friday that it has received an unsolicited, non-binding buyout bid from private equity adviser SQN Investors, valuing the software firm at A$441.9 million ($300.62 million) ($1 = 1.4699 Australian dollars) (Reporting by Tejaswi Marthi in Bengaluru; Editing by Shailesh Kuber)