Hi Isanchez, thanks for your input. Your experience with hum sounds similar to mine.
I think this finding of hum could be somewhat system-dependent. Not that the 324 only has hum in certain systems (although it's possible that certain 324's have more noticeable hum than others, and cartridge output level will definitely play a part in what is heard at typical listening volumes). But I suspect that perhaps the hum can be more noticeable in higher-powered, multi-way, full-range system contexts than it might in some lower-powered systems using smaller 2-ways or single-driver speakers (dynamic or electrostatic). At least this is what I took from trying my 324 in my single-driver bedroom system: the hum didn't disappear -- in fact I doubt it even diminished. But it became much less noticeable, both because that system isn't as highly resolving or extended at the frequency extremes, and can't be played anywhere near as loud.
About your impression that the 324 adds some benign distortion, I haven't heard this as such. I do find that using the output transformer gain-attenuation switch set to -6dB or -12dB can usefully tweak the harmonic flavor, in the fatness vs. tautness of the bass, tonality of the mids and sharpness of the treble, dependent on choice of cart, which step-up tap is selected, and in combination with the partnering preamp. Noticing this made me wonder that the output transformer ratio setting could likely have an effect on output impedance (and possibly even somewhat on the reflected MC input impedance?).
I haven't heard the Einstein, but I suppose your take that the 324 sounds more like legacy tube gear could have something to do with the amount of coils and iron used in the 324: input and output transformers, with inductors in both the power supply and the amplifier circuit. The 324 does not sound rolled off at the frequency extremes to me or anything like that, but unsurprisingly it does sound most transparent and neutral when used in MM mode, skipping the step-up transformers, though not as forceful or dynamic with my medium-output MC cart.