What isolation feet to use??

Make sure you have a nice slab of hard Maple for a stand. I use a welded wood i.e. flat grain butcherblock but I understand that a well cured thick piece of solid wood is better. There is some debate over edge grain Vs flat grain but to my knowledge end grain is not the best. It is the easiest to find in thick pieces. My amps are on 3 " thick stands which themselves are footed with small comparitively speaking brass cones/spikes into discs.
Check out Mapleshade for ideas. The Edensound person here on the gon has some of the best prices for massive brass feet.
Personally I have used very a un-WAf like home brew memory foam pillow under my preamp with astonishingly good results. I was hoping to get a wood "sand box"and use dense closed cell memory foam layers alternating with layers of tungsten or steel coated lead shot with maybe some big heavy feet. Brass looks like gold because some places actually do electroplat the brass with gold quite the show.
The sound may get overdamped though which would not be so great. If that happens you know it everything becomes a dull thud.
Speedy metals on line says that small orders can be accomodated and I guess you can buy shaped rods of various alloys and have them fabricate what you want.

Having listened to many...I like Dead Ball isolators. The dealer I bought them from offers a money back guarantee.

I'm not sure about the audiophile stuff, but isolation in physics is a kind of decoupling, and the purpose is to absorb or cancel out vibrations. The ideal model for this is the spring and dashpot, the next best thing is a visco-elastic material. You need to have deflection in order to decouple; imagine a spring that "bounces" at the frequency of the vibration you want to isolate. On the other hand, if you apply pressure against a relatively hard/inert object, it absorbs none of the pressure, and actually transmits that to the other side. As an experiment, hold a brick and have someone push you, versus a pillow, and see which absorbs and deflects more pressure.

The same principle applies in audio. Rubbery visco-elastic materials, springs, dashpots, shock absorbers, hydraulics, tuned mass dampers, etc. are decoupling devices. Wood, brass, silver, metalic, and any hard surfaces, like compressed sand or shot, are coupling devices. Coupling devices will transmit vibrations from your equipment to the ground, and from the ground to your equipment. There's no such thing as having only one-way (Newton's law: for every action there's an equal reaction in the opposite direction).

I'll treat lightly, since I don't want to offend anyone, but there are a lot of ideas here that flies right in the face of science. For some no-nonsense literature, for one, go to sorbothane . com and read what they have to say, and they even have a nifty program where you plug in a couple parameters according to a formula, and figure out the ideal size isolation feet you need for the project (hint: depends largely on the weight, aim for the lowest frequency). Sorbothane manufactures a proprietary visco-elastic material that many of these other audiophile products source from.

Note: I have no affiliation with Sorbothane, but the program on its website really is just the easiest way to figure out the idea feet for your equipment

Funny how many approaches there are to coupling/decoupling/damping/etc. I personally hate sorbothane and firmly believe in Mapleshade's brass cone/maple platform (not butcher block)/Isoblock approach, but then I watched Pierre Sprey demonstrated the whole process before my very eyes (and ears) in my own system, so I know how well it works. Along the way I've also tried sand boxes, inflatable innertubes, bubble wrap (!), and dead balls (not bad at all), not to mention sorbothane (ugh). So, in all particulars, YMMV. Dave