I've watched almost every episode of Lee Mujin Service, and I think you're not wrong in thinking he was extra impressed in this episode. I think he does just love hearing people sing, and he's always loving it. But I don't think he's been as speechless in the other episodes as he was here.
I really enjoyed how he reacted to Sieun from StayC as well. He kept insisting that she has perfect pitch throughout the episode.