The beam quality of large-mode-area fiber amplifiers was investigated at the 10 W power level using a tunable ring cavity, that is also used in the laboratory system of GEO600 as a pre-mode-cleaner for mode filtering. More than 98% of the overall output power were contained within the polarized (200:1) TEM00 mode with an appropriate choice of coiling diameter. With the high sensitivity ring cavity analysis, the beam quality improvement caused by decreasing the coiling diameter was verified, while this could not be seen within conventional M2-measurements. The results are compared with the properties of single-mode fibers.