The high-gain settings on the Spider IV are pretty "gainy" sounding right away, whereas the Flextone is much more elegant in it's higher gain tones.
I would personally not use the "insane" or plexi settings. The insane doesn't have as much definition as a recorded Van Halen tone, and the Plexi tones on the Flex are very realistic...that is to say it sounds like a real Plexi! You have to open up all the EQ knobs to get it to bark...don't be shy! But Eddie's sound is so processed on the early Van Halen records, they do not sound like a Plexi to me. Maybe a plexi with a distortion pedal in front or something? Ted Templeman really tweaked the tracks all to weirdness.
Rather I would use the JCM 800 or 2000 amp models on the Flextone. Don't be afraid to put the bass, mid, and treble controls on max on your flextone...that can really change the character of the distortion on the amp models. And if you want the appearance of a larger sound, dropping the mids a bit can make the amp sound louder or heavier.
Lastly, when you think you have the gain where you want it, try switching cab models. I assume you know how to do this? (Press on the amp encoder so that the LED shows green, and then turn it while pressing down.) Switching cabs is a great way of using different EQs on amp models...you can take any amp model and run it through any imaginary cabinet. (Be careful here - if you hold the encoder down on a "green" cabinet model for around 3 or 4 seconds, it will flash and that cab model will be permanently attached to your amp model...a customizing feature.)
Don't be afraid to try different amp models too. Some of the best higher gain amp sounds I have made are using the Rectifier models with different cabs. And, if you are playing loud, turn your drive down!
I hope this helps in some way.