It makes total sense that the Szerkeres would sound great. The principles for what make a good Grado amp should apply to the Magnums. That is to say, voltage swing is secondary to high current. The Szerkeres is, as far as I know, the amp that can push the most current. Most Mosfet based output stages can drive low impedance cans well. I'm not sure about commercially available builds, but Millett Starving Students and Mosfet Maxes should do well, too. Another great option, more or less designed for Grados is the Torpedo.
That said, the Szerkeres is technically not an amp, but rather a slightly-less-than-unity-gain buffer. Amps that sound slightly inferior with Magnums could easily be retrofitted with the Szerkeres, which would reduce the voltage swing slightly, and for higher-impedance cans may hurt sound quality, but for low-impedance cans should maximize current.
Regarding vintage gear, a lot of it has been found to sound great with low-impedance cans. I have a feeling that a lot of that stuff was able to have massive output currents, and even with the resistors off of the speaker outputs, the amount of current is still sufficient to sound superior.