Do they have problems with the snare, or is it really just the kick? I'd be willing to bet that half the problem is from the pedal stomping rather than the actual kick drum sound. Actually, I just realized you said you already have mute pads on, so it's probably a little more than just half the problem. They probably can't really hear much of the actual drum kick sound, they just experience shake, shake, shake.
I see people suggesting electronic drum kits; however I feel these will do you no good as most good ones use real kick pedal apparatus with a mallot that strikes a stopper pad/sensor. Also, an electronic drum kit is essentially nothing more than mute pads with sensors, so if you have a problem now an electric kit won't solve it.
If you have high enough ceilings you could do a quick and dirty drum riser by placing masonry cement bricks next to each other in maybe a 9 brick x 9 brick pattern. Maybe more, idk, but if you look it up the VERY best thing for blocking sound is cement brick. Just as good as noise cancelling foam, if not better. Also, I believe, better than attempts to "float" the floor.
While it's not floating, it will probably help with the direct transfer of the vibration of using the pedal as well but if you could suspend yourself with cables that'd be ideal ;).
Of course, I don't know how crazy you are willing to get, but you could conceivably enclose the entire bass drum and kick pedal apparatus in a sound-proof enclosure, with some sort of access panel/door that lets your leg in so you can press the pedal.
However, since I don't really think the problem is in the sound transfer so much as the vibrations caused when playing, your best bet would be the cinderblock riser. If that's too heavy for your floor, then your gonna have to either move or drive your neighbors to move out. :) Hope this helps.