Isolation device under Ayre CX-7e?

Check out musicdirect.com for the factory recommended Ayre Myrtle Wood Blocks. These are the Cardas (manufactured for Ayre) blocks which do the right thing to support your unit.
Otherwise, my default support recommendation is to check out Herbiesaudiolab.com for Tenderfeet, or Big Fat Black Dots, either of which represents a cost effective great support idea.

Hvu, You can compare the use of Herbie's footers in two different ways, first directly between the Ayre CX-7e and your maple board base, and second under the maple wood base.
I think directly under the component can be the most neutral, allowing a component to be at its best.
Incidentally, I have found that placing one or more Cardas/Ayre Myrtle Wood Blocks on top of a component, especially the wooden plinth of a turntable, can fine tune the sound separately from the overall benefit of any footers.
With any support method, there can be an adjustment period of minutes to hours before the completed benefit, or effect, upon a component is audible. This endpoint is when there is a sense of ease or rightness to the sound, without any listener fatigue.

Hvu, It is extremely stable with three footers under a component case, and Herbie's or Cardas/Ayre footers can bear great weight and still provide desired vibration control.
You place two of the footers under the heavy side of your component, and a single footer under the lighter side, if there is a big difference in distribution of weight to be supported.
If each Herbie's Tenderfoot is not tall enough when you do not remove the original factory feet then you can simply place a Myrtle Wood Block footer beneath each Herbie's Tenderfoot.
You will be surprised how you can make subtle adjustments in how your CX-7e sounds when you use different footers alone, or in combination. These are all very reasonable in price.

A paper coaster is a small round piece of absorbent paper on which a drinking glass can be placed.
MDF is usually not as good sounding as wood, depending on the footer. But, with Cardas/Ayre myrtle wood cuboids the ultimate sound varies with the positioning under the component, as well as the underlying flat surface material, so you could start with MDF if it is already in your possession.
I have experimented in different systems with the Cardas myrtle wood cuboids ("block" is not a fancy word when advertising this item) on different pieces of wood furniture, on maple slabs, on granite slabs, on formica surfaced plywood, and even between two stacked components (one should never admit this) with satisfactory results, because there are so many variables.
These cuboids can be placed on top of component cabinets to damp the component, and change the sound in a subtle way, which can be better or worse, so you just have to be flexible with this sort of inexpensive, helpful "tweak".
Sometimes, there is no precise "right or wrong" answer to a question which makes it possible for you to decide for yourself, and not have to feel you did something "wrong".