The interaction between aging oceanic plates and their underlying mantle is a crucial component of the plate tectonic cycle. Sub-lithospheric small-scale convection explains why plates appear not to thicken after a certain age. Yet, many open questions still surround this process. Here, we link grain-scale processes, dynamic models of asthenospheric flow, and seismic observations to gain new insights into the mechanisms of small-scale convection. We present high-resolution 3D geodynamic models of oceanic plate evolution using the community modeling software ASPECT. These simulations use an Earth-like rheology including coupled diffusion and dislocation creep as well as their interplay with evolving olivine grain size. Our models quantify how the balance between diffusion and dislocation creep affects the morphology and temporal stability of small-scale sub-lithospheric convection, including its onset age, the average depth and wavelength of the small-scale convection rolls, and the amplitude of the temperature and grain size anomalies within the rolls. We directly relate these quantities predicted by the dynamic models to geophysical observables through laboratory-derived constitutive relations, converting variations in temperature, pressure, grain size, water content and stable melt fraction to seismic velocity and attenuation. By creating synthetic seismic tomography models of different dynamic scenarios and comparing them to observations from the Pacific OBS Research into Convecting Asthenosphere (ORCA) experiment, we determine the parameter range in which geodynamic models fit these seismic observations. This provides new constraints on oceanic asthenosphere rheology beneath this part of the Pacific Plate, with potentially broad implications for Earth dynamics.