UPDATE 6-Omnicom, Publicis call off proposed $35 billion merger
* Merger of equals founders over power struggle - Levy * Tax, regulatory issues had held up closing * Analysts see tougher times, M&A ahead for ad sector (Adds source details) By Anjuli Davies, Soyoung Kim and Leila Abboud LONDON/NEW YORK/PARIS May 9 (Reuters) - The $35
