Regarding the DC gain, I looked into a sallen key filter before, low pass, and I can't exactly remember why, but if you look at the transfer function, if you introduce a DC gain greater than 2 or 3, you get a right hand plane pole, and it becomes unstable. I wouldn't go any more than 1.5, or you'll get a bit overshoot and possible instabilities.
I needed a gain of 15, so I just had a unity gain sallen key filter, and then a gain stage at the end - this was sufficient.
Hope it helps!