Everyone is going to recommend what they found worked for them. I’ll tell you with no reservation that you HAVE to try the product with your speakers to know if the specific synergy is there. I, like you try to buy used so I can take my time listening with the option of reselling with little or no loss. USAM has no seller fees btw. I tried 5 or 6 different amp/pre-int’s on my speakers and 1 int specifically made the speakers wake up and sound great! When those speakers got old and I replaced them the int that was so magical with the last speakers didn’t have the same effect on the new speakers.
Of your 2 options I would def expect the mufi to be the safe choice because they have a good rep, are more likely to make you happy and if not should resell pretty quick if the synergy isn't there. What speakers are you driving?