Why MDF so Bad???

do you think I'm better putting the rack directly in the middle of the 2 speakers or leaving the rack a bit behind the speakers...

I would think that the rack placed somewhat behind the speakers is the more typical approach, but with the rear firing woofers you might find it works better otherwise. However when placed directly between speakers, staging may be degraded.

your experiment is very interesting & I believe that you may be pleasantly surprised by the results. I used a similar rack arrangement for many years & have always liked it. Not sure where your perception that MDF is undesirable came from, but I disagree with that conclusion based upon my own experience. I don't think that you'll need to treat the shelving at all. My shelves weren't even that thick: closer to 5/8" is all that mine were. And the whole thing was assembled from pieces in a Sauder Furniture kit; fastened with steel screws & the shelves sit on adjustable side pegs that plug into a column of holes such that the shelving heights are adjustable. Quite solid & rigid, it doesn't even have spikes which (I'm guessing) may not be very effective for this type of high mass rack anyway. Rather it has casters which are fantastic for rolling away from the wall & accessing the back of equipment for cabling changes or tweaks, but spikes could be installed in place of the casters (I have the spikes on hand, but I love the casters for convenience). It also had smoked glass doors which looked great, but I eventually removed them because I figured that it sounded a little better without them resonating, at least that was my perception. If you're interested in trying this rack it's now available cheap; it is shippable. I only replaced it because it has 7 shelves & I need 10 for my larger system now. It's double wide & even has compartment storage for records or accessories.
So I think that you'll have no problems with your new rack. You might find for certain pieces of equipment that audiophile shelving such as Black Diamond, Zoethecus, Symposium, Neuance, etc. placed atop the MDF shelves still enhances performance. Pods or cones placed between the MDF rack shelf & the audiophile equipment shelf sometimes enhances or even degrades sonics, or has no perceptible effect at all, so experimentation is required. Same applies for cones, footers, etc. placed between the components & the MDF shelf or the audiophile shelf when used. The whole thing responds quite well to these kinds of tweaks. My new larger rack is also made of wood, which I find much more attractive & WAF-friendly than black steel etc. Good luck with your project!

Hi Bruce et al thanks for your valuable input.
I fully agree with the "posting our experiences" philosophy.
Zargon: from what I've read (I have not tried it myself) the long grain should run horizontally for best results with maple vs. the vertical grain butcherblock. The Mapleshade people also agree with that approach FWIW.

Both Roy's & Bruce's comments above are very interesting & in fact are applicable & valuable info. regarding my latest rack issues. I've recently observed that when changing my equipment from the old veneered MDF all wood cabinet to the new cherry veneered (+ ? wood, I don't know what kind of wood is inside there) shelved Target rack (which has steel tube framing) that the initial results weren't quite as desired. I much perferred the sound of the older higher-mass cabinet. Many of the previous tweaks that I'd 'perfected' no longer do the job on the new rack, & in fact I've determined that the Vibrapods topped by Black Diamond shelves topped by brass or titanium cones are now actually making matters worse.
So to experiment, I am going to get a Neuance shelf & a Mapleshade shelf + probably some of their other cones & IsoBlock footers etc. Last night I placed mini tennis balls under the preamp, which helped to restore the now-thin midbass, but hasn't yet fixed up the low bass back to what is was previously; I'm going to try some superballs tonight instead. Also the highs aren't as scintillating as they were previously, so I still have a lot of work left to do. Fortunately at least the CDP still works prety well on a Zoethecus Z-slab, with Vibrapods underneath it & Nordost titanium Plusar Points above.
If all else fails then I'll either go back to my old cabinet (a last choice for space limitation reasons only) or I may look for someone to build me something similar to my old cabinet, but with more shelf space. Bruce I don't know where you're located but it sounds like you might be the perfect man for the job, if you're willing?

thanks for the suggestion Les: I believe that material is called HDF, for high density fiberboard