DIY TT Stand on concrete floor - I'm confused

I've read countless articles and posts, and I'm as confused as ever. I am ready to build a stand for my turntable only. All other equipment will be on a separate unit. The floor is concrete (basement) covered with a 1/2" pad and fairly deep pile carpet. My tt is a Thorens a TD-280 Mk2, which is unsuspended. It's been modded with an MDF base under the plinth and modeling clay lining the underside of the plinth to deaden it. My current plan is to build a rack with (3) 4" OD sand-filled PVC tubes supporting a sand box made of MDF. The table would sit on metal spikes directly on the top of the sand box. The base of the stand will be coupled to the concrete floor by spikes through the carpet. That's where my confusion started as I've read to both couple to the concrete for equipment yet not couple to the concrete for speakers.

Since it'll be on concrete, I think my plan will definitely be vibration proof. But will it affect the sound, and how? Any suggestions would be appreciated.

I would couple to the concrete floor with spikes, rather than sit on a pad and carpet. You want as ridgid a support as possible.
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All sounds good as to what you are doing with the stand and the coupling to the concrete floor. But on the other hand, since your turntable has no suspension, the only thing that would do differently (or at least try) is to replace the spikes on the turntable with Herbie's Tenderfoot isolation feet. Those things really work and they are not expensive.
I don't know about spiking the rack versus spiking the speakers. I do both
and find it works in my room, though I don't have concrete floors. In
addition, you may be immune to most vibrations, but in my experience, you
will not know for sure until you place your unsuspended turntable on a
superior isolation platform like a Vibraplane and compare that to what you
have. These are used for very sensitive lab equipment like lasers and
microscopes. These labs often have concrete floors with no detectable
vibration, and yet the equipment still benefits from being placed on an
isolation platform.

I'm certainly not a guru nor do I sell these. And I'm not writing that this is the
only way. It is simply the best way that I have found to achieve what you are
trying to do. I've heard the benefit for my former unsuspended turntable and
now for my SS amps.

I think you should complete your project and get to know the new sound.
Then if you have a chance, try to borrow a bench top air isolation platform to
see if you hear a difference.
Do you live near any busy roads ? How about areas of construction ? These situations can cause your cement slab to absorb and transmit vibrations to your equipment . Heavy trucks especially can cause this . Some of the vibrations may be absorbed by the surrounding soil if your basement is suffiently under the surface .
I decided to put soft rubber caps on the bottom of my steel posts and then set them in floor protectors used under furniture legs . This all sits on padded carpet on top of a slab on grade .
As mentioned above , try both ways and see what develops .

Good Luck
I actually have some absorbing pads. I'll try it with the spikes and the pads and see which I prefer. As for vibration from roads and heavy equipment, that's not an issue. I'm pretty rural, and on a couple acres of land. I appreciate the thoughts and suggestions!