We like Vivid and some older "Vintage" stuff from (cringe) Adam & Eve Video. We have collections of porn (on DVD) from the early days of the movie camera to modern day. We also have some older Caballero vids, from the 70s and 80s, and My Man has some stuff from Europe on VHS. I don't know the studios, though. Both Nina Hartley and John Holmes used to work for Caballero. I don't think they ever worked together though. I think Nina was just getting into the business around the time John Holmes got arrested and then found out he was HIV positive.
Did you know that some of the first things committed to film was people fucking? Yeah, it's that interesting. The first "movie" ever showed in movie houses (which were then play houses or Vaudeville Stages) was a movie, by Thomas Edison Films called "The Kiss." It was a LONG kiss and people wanted it banned. The entire movie was just a man and a woman kissing rather passionately.