How important is an audio rack?

The audio rack/stand/isolation ante is much higher with turntables in particular than with other gear. Other mechanical devices, like CD players, maybe.

For amps, pre-amps, etc., I'd focus on isolating each component from external EM or RF noise/fields, including those from other components. Low level phono circuits are also most susceptible to these as can be CD players and any line level gear.

The easy solution is to provide space between components, especially between phon and line level devices and power amps. ALso keep away from eletrical appliances, etc.

PRoper isolation is important for really good sound. Its often not as complicated or need be expensive as some may make it seem.

I think the rack or shelves can make or break the system. I've not come to to a definitive conclusion on what is best but I'm leaning towards MDF over sand doesn't sound that great. It's a tough thing to do A to B comparisons. I have two grand prix racks for my living room and bedroom systems. I have been doing some remodeling and re decorating and I really don't care for the look and size of the grand prix and would like to build a nice looking wall mounted shelf unit to hold all my gear but I'm afraid getting rid of the grand prix rack might make my sound suffer.

I have a 72" Florence Knoll Walnut Credenza that I inherited from my father who bought that and a matching shorter credenza that I use as a sofa back in my living/ listening room. It is classic mid-century design at it's best and fits beautifully in my 1916 Chicago Bungalow. See it in Virtual Systems.

My experience, having been down this road with DIY and my store-bought audiophile rack is briefly:
1)The shelving material is a lot more important than the construct/rigidity of the stand.
2)MDF is terrible.
3)This is a very particular area where people get sold theories and believe whatever they want.
4) Light 'n rigid has worked well for me, and not using a good rack is lost potential. Footers are not worth the money, IME for my system.

Caterham1700 has some good posts on the subject.

My experience with racks is minimal to the moment, but i think i need to try some of the big buck ones. That's where the improvements are audible from what i know.