We report on the experimental realization of two different low-loss single- to multimode fiber coupler designs, each combining more than 5 kW of output power. The adiabatic mode evolution in the seven-fiber input bundle provides a brightness close to its theoretical maximum. The all-glass structures exhibit very low heat loads mainly due to small avoidable contamination, indicating room for improvement. To the best of our knowledge, regarding all-fiber geometrical combined power generation, beam quality and heat stability, the presented results had never been reported before.