Resonant enhancement of spin Seebeck effect (SSE) due to phonons was recently discovered in YFormula: see textFeFormula: see textOFormula: see text (YIG). This effect is explained by hybridization between the magnon and phonon dispersions. However, this effect was observed at low temperatures and high magnetic fields, limiting the scope for applications. Here we report observation of phonon-resonant enhancement of SSE at room temperature and low magnetic field. We observe in LuFormula: see textBiFeFormula: see textGaOFormula: see text an enhancement 700% greater than that in a YIG film and at very low magnetic fields around 10Formula: see text T, almost one order of magnitude lower than that of YIG. The result can be explained by the change in the magnon dispersion induced by magnetic compensation due to the presence of non-magnetic ion substitutions. Our study provides a way to tune the magnon response in a crystal by chemical doping, with potential applications for spintronic devices.