No matter what - rehearsed, under-rehearsed, over-rehearsed, doubts about rehearsing - the first gig is always the first gig, and you put on your little praying hat, batten down the hatch, and do what you do.
If you look at a record under a microscope, the high frequencies are short jagged edges... and the low frequencies are long swinging ones are deep bass sounds. When it cut it at half speed, you're getting more of those on the record.

