One synonym for "bassy" is "low-frequency," referring to the physical characteristic of sound waves with a longer wavelength. Another option is "deep," which can also describe the tone or timbre of a sound. "Boomy" or "thunderous" might apply to particularly powerful bass tones, while "resonant" could describe the way bass notes reverberate or linger in a space. "Rumbling" or "vibrating" suggest movement and energy. Finally, "subsonic" technically refers to sounds below the threshold of human hearing, but can also be used to describe extremely low tones.