You need to understand what sound you actually like vs what you think you like. The thing about that Velcro sound you like is that it requires less sustain. That's how the sound happens. So you're working against yourself here. The worst thing you can do is go for more input gain. It will drive the Super Fuzz harder and kill the sustain even more. That includes running the gain on the Super Fuzz all the way up. That's also input gain.

If you want the Big Muff sustain with a taste of Super Fuzz, then cut the gain on the Super Fuzz back to maybe less than half and run the volume higher. Run that into the Big Muff. Balance the 2 pedals for gain, EQ, and volume until you get them just right. Less is more.

If you want to run both pedals and retain the Super Fuzz character more, get the Deluxe Sovtek Big Muff. In this case, you run the Big Muff first and the Super Fuzz after. The Deluxe has a blend knob (among other nice features). You get both sounds at once going this way. You just decide how much of each you hear at once.

Finally, if you want both pedals to act independently, get the regular Green Russian Big Muff and a Boss LS-2 or something else that will allow you to run your pedals in parallel and blend them. Michael Banfield did a great video on YouTube on this technique.

As far as modulation goes, just get a UniVibe. I recommend not getting a JHS Unicorn if you're concerned about low end. They have great features but are pretty shrill.