Soooo....
It turns out that the monologue sample for Sixtyniner and Sir Prancelot Brainfire is from the adult film "Summer of '72" https://www.imdb.com/title/tt0128647/. Definitely has been the most awkward sample to sleuth, but thankfully it didn't take too much investigation.
BUT...it also turns out that "Summer of '72" was the source for the background music/love scene samples in Niagara and P:C:.
AND...there happens to be a character duo named Julie and Candy in the film, which is too good to be a coincidence
Also the music during the monologue sample is eerily similar to the melodies found in The Color of the Fire /I Love U. It feels like the music especially during the beach sunset and fireplace scenes must have been very influential. It reminds me a lot of Bruce Langhorne's works.
Anyway, pornographic content aside, it really is a lovely monologue and there are some really nice cinematographic gems here and there. For your convenience I uploaded a short video montage with the audio sources of the BoC samples (Disclaimer: there's NO NUDITY but obviously contains ADULT THEMES).