Incidentally, you can synthesise an inductor with an op-amp. So in principle you could produce the same filter curves as eg the 33, but actively. Then that would get around the problems of magnetic core non-linearity and hum pick up (that in the 33 was difficult to eliminate entirely). The (gapped) pot cores in the 44 were better in this regard.
See for instance part way down the page here
https://en.wikipedia.org/wiki/Gyrator
Craig