An audio preamplifier includes a first stage including a first triode of a first vacuum tube in a common cathode configuration configured to perform a first gain function favoring one of low frequencies and mid-range frequencies in response to user input; a second stage including a second triode of the first vacuum tube in a common cathode configuration configured to perform a second gain function favoring one of low frequencies and mid-range frequencies in response to user input; a third stage including a first triode of a second vacuum tube in a common cathode configuration configured to perform a tone-shaping function favoring one of low frequencies and mid-range frequencies in response to user input; and a fourth stage including a second triode of the second vacuum tube in a follower configuration configured to perform a tone stack function in response to user input.