Specialty fibers such as chirally-coupled-core fibers (3C®-fiber) show a high potential for further power scaling of single-frequency fiber amplifiers. Especially, the application of gravitational wave detectors requires a high optical output power at low noise characteristics. The output power of fiber-based single-frequency amplifiers is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In general, to reduce the impact of nonlinearities, the mode area of the fiber core is enlarged and 3C®-fibers have been specifically designed to enable single-mode operation with a large mode area core. This fiber type consists of a step-index fiber structure, whose signal core is additionally chirally surrounded by one or more satellite cores. Because of the phase matching and the helical geometry, the higher order modes are pulled out of the signal core, which allows a high-purity modal content in the core. The development of compact all-fiber lasers in conjunction with specialty fibers combines the advantages of both techniques. For the first time, we demonstrate a spliceless all-fiber amplifier, where all optical components are directly integrated in a single Yb3+-doped 3C®-fiber. Such a spliceless laser design allows a compact and robust architecture using specialty fibers, while maintaining excellent beam properties. At an output power of 336 W, a fundamental mode content of 90.4\% was demonstrated. This work emphasizes the suitability of 3C®-fibers in high-power laser and amplifier systems and the potential as laser sources for the next generation of gravitational wave detectors.