Single-frequency Er3+:Yb3+ co-doped fiber amplifiers (EYDFAs) are promising candidates for laser sources in the next-generation of gravitational wave detectors. The high power scalability of EYDFAs can decrease the quantum shot noise while the wavelength around 1.5 µm is the most favorable for cryogenic cooling of the optics to reduce the thermal noise. In this work, we present the recent progress on a fully monolithic, 2-stage single-frequency EYDFA that utilizes polarization-maintaining fibers. We present a comprehensive study on different pre-amplifier concepts with a seed input power of 8 mW and sub-MHz linewidth. We discuss the limitations, i.e. ASE, SBS or technical issues, and demonstrate that cladding-pumping with 940 nm provides the highest gain without the onset of ASE and a maximum output power of 1.07W. Furthermore, we demonstrate SBS-free operation of the pre-amplifier by relative intensity noise (RIN) measurements. The pre-amplifier was on an engineering-ready level, i.e. possesses temperature control, monitoring and housing. The pre-amplifier was long-term tested and characterized with regards to its noise properties. The high-power amplifier utilized an Er3+:Yb3+ codoped and polarization maintaining LMA fiber. The high-power amplifier was also pumped at 940 nm in counter-propagation direction. An additional cladding light stripper was introduced at the output to eliminate residual ASE light from the cladding. The high-power amplifier provided an output power of 110W in a Gaussian-like mode, had an ASE extinction ratio of > 50 dB and only marginal Yb3+ ASE power levels. We show that the amplifier operated SBS-free and discuss the polarization, i.e. PER, and long-term performance, i.e. cooling requirements.