I heard an interesting report from Colin Kaminski during our Water Panel presentation at NHC this year. He said that hop expression is muted as kettle wort pH is reduced.
I had not put this effect together before in my mind. But I had noticed that some low pH worts that I had produced in years past, did not seem as bitter nor hoppy. I just never put the cause and effect together before. So I think that Colin is correct that there is another effect of too low wort pH and that is low hop expression. This is in addition to increased attenuation and tart flavor perceptions when wort pH is lower than ideal.
So if the overall pH of the pre-boil wort in the kettle is less than about 5.2, you might experience this reduced hop expression. It may be better to keep pre-boil pH around 5.4. Of course, these are all room temperature pH measurements.