Just to let you in on a little secret...
These mix-tapes were all made by rave DJs.
The tapes were almost entirely made using 2 turntables, a mixer and a shitload of dope 12"s.
That's about it really.
I used to go to a lot of raves and see these DJs spin their shit. Sometimes they would put their own records in the mix. Sometimes, they would get special test-presses or dub-plates from well known producers and either drop them at the raves or throw those on the tapes.
As far as what the producers used to produce the tracks: software, midi, samplers and synths. They had all of that in the 90s, it was just much more basic than it is now.
The tapes were a big part of the rave culture. I collected a lot of these at the raves. Often times, you would go to a rave and there would be a table set up with a fuck-load of mix-tapes sorted by genre and you would just buy the ones from the DJs you knew and liked.
Gone are the days...
If you really want to find out more, start by researching some of the record labels. Most of this mid-90s shit was produced in the UK.
The best labels, in my opinion, are:
Suburban Base
Dope Dragon
Joker
Ganja
Congo Natty
Moving Shadow
V Classics
Hard Leaders
Promo
Fresh Kutt
Metalheadz
Some labels like Hard Leaders, Metalheadz and Moving Shadow had darker stuff but it all varied from label to label, artist to artist and track to track. There are literally countless labels from this time period that put out quality shit. You are in for an exciting hunt.
The vast majority of producers were also DJs.
If you like dark, Technical Itch is where the buck stops. He is the best.
If anyone knows if he ever cut a mix-tape, I would gladly blow you for a lead... but I doubt he ever did.