Phase stability of polymer nanocomposites (PNCs) composed of polymer-grafted SiO2 nanoparticles (NPs) blended with high-molar-mass host polymer chains is investigated. We focus on blends in which the particle-grafted polymer, polyethylene glycol (PEG), and the host-atactic poly(methylmethacrylate) (PMMA) or PMMA/oligo-PEG blends-exhibit favorable enthalpic interactions. Small-angle X-ray scattering measurements are used to evaluate the phase stability of the blends and to report on the structure of the materials at intermediate and long length scales. By exploring SiO2-PEG/PMMA and SiO2-PEG/PMMA-PEG systems covering a wide range of molecular weights (M-w) of PMMA (1.1 kDa <= M-w,M-PMMA <= 1.1 x 10(3) kDa) and tethered PEG (0.5 kDa <= M-w, PEG <= 2 kDa), we are able to develop a comprehensive stability map for PNCs based on hairy NPs. At low M-w,M-PEG, the phase behavior is dominated by entropic effects and the negative Flory-Huggins chi parameter between PEG and PMMA plays no role in phase stability. For higher M-w,M-PEO and intermediate M-w,M-PMMA, a crossover from entropy- to enthalpy-dominated behavior is observed, which leads to the phase stability in PNCs well beyond the conventional limits reported for SiO2-PEG/PEG mixtures. This enhanced mixing ceases above a critical M-w,M-PMMA, where it is found that PMMA chains wet a sufficiently large number of SiO2-PEG particles to bridge and thereby destabilize the composites.