* Shares trading far below offer price (Recasts to include investor reaction, adds shares, analysts): SYDNEY (Reuters) - Australian adult education provider Navitas Ltd on Monday rebuffed a A$1.97 billion ($1.4 billion) approach from its founder and a private equity firm, leaving some investors concerned the tactic may not generate a better deal. Navitas said in a market filing that although the existing BGH-Jones price tag undervalued the company, it was willing to negotiate and give its suitors a “detailed management presentation” rather than unfettered due diligence.