You can't isolate speakers because you can't isolate sound. Assuming your speakers are on the floor, if you have a resonant floor and the speaker excites it then you have to dampen the floor with carpeting and/or stiffen the floor.
Subwoofers that are not balanced have to use spikes or they will start walking. Balanced force subs you could put on roller skates and they would not move.
Putting isolation platforms, most of which do not work, under anything except turntables is a silly proposition based on lay instinct which is usually badly mistaken. But, it looks cool and that's really what it is all about. The two isolation platforms that actually work are the MinusK and the Vibraplane if you need one for your turntable.
Good for you building your own speakers! Check out the new DEQX Pre 8 digital preamp. It has a 4 way digital crossover in it, totally programable for xover points and slopes. Could be the best thing to happen for DIY speaker builders, instant perfect crossovers.