What is the opinion of Reddit about the
S.M.S.L SP200 Headphone Amp THX AAA 888 Technology XLR RCA Balanced Headphone Amplifier?

It sounds like you are getting really deep into this hobby. I would suggest you save yourself some money in the long run, and just get an end-game (or close to it) tier amp / dac like the SMSL amp and dac or the Monoprice ML DAC/amp combo (it goes on sale for 500 every so often). Take it from lots of us, the mid-fi hunt can be an expensive waste of time searching for your end game.

Yayaya https://www.amazon.fr/S-M-S-L-AAA-888-Amplificateur-casque-%C3%A9quilibr%C3%A9/dp/B07ZNDN5PF/ref=mp_s_a_1_1?dchild=1&keywords=thx+789&qid=1632957373&sr=8-1