i have the current f/r lta zotl40 ref power amp, running various el34’s, i also have several arc power amps (ref110, ref75, and so on) but not an i-50
i would venture to say that my guess is that the i-50 will give a somewhat more overtly tubey sound signature than the zotl - which, because it does not have standard output transformers between the power tubes and the speaker load, has almost a solid state like degree of clarity, speed and frequency extension (but with better imaging, and more delicacy around voices) - also its el34 power tubes would have a touch more finesse than the 6550/kt88’s in the arc unit
neither will have anything near the bass grip of the pass xa25
hope that helps