Emerging applications, such as gravitational wave astronomy, demand single-frequency lasers with diffraction-limited emission at 1.5~μm. Fiber amplifiers have greatly evolved to fulfill these requirements. Hundreds of watts are feasible using large-mode-area and specialty fibers. However, their application in a few watts to tens of watts in monolithic systems is unnecessarily complex due to the poor commercial availability of fiber components and standard integration procedures. In this Letter we propose and experimentally demonstrate a novel and simple method to amplify single-frequency signals at 1.5~μm up to tens of watts by core-pumping single-mode Er3+:Yb3+ fiber amplifiers at 1018~nm. The proof-of-principle system is tested with different active fibers, lengths, and seed power levels. Over 11~W with an efficiency of more than 48\% versus launched power is achieved. Additionally, performance degradation during operation was observed for which photodarkening due to P1 defects might be an explanation.