The Avalon is an extreme close analogue replication of the TB-303, which comes with additional features and impressive built quality. It sounds closer to the TB-303 than my TB-3 or my X0xb0x. Hands down, the best TB-303 clone out there, right now (Nov. 2016).
Avalon is a wonderful analog bassline synthesizer, with the exact same components and circuitry of the Roland Transistor Bass 303. Avalon also has an optional second layer of features that improves upon the limitations of the TR 303, including a sub-oscillator and Eurorack compatibility. Love syncing this with my Arturia DrumBrute or my Moog Sub 37.