@bluethinker thanks again for the useful response, much appreciated. From your description it appears that the Diablo 120 may be voiced slightly to warm or soft in the high frequencies since it matches well with speakers which have a dominant or hot treble. I won’t be able to live with an amp that has a warmer or softer treble than the already warm, soft and delicate Luxman 590AXII.
As I’m buying used, I won’t be able to try but will likely go with either 300 or 120. Another available option recommended to me is the Audionet Watt but I’ll pass.