spring loaded platform non-suspended turntable

There is no question that your SP10 would benefit from some kind of isolation. Minus K has the advantage that it is mechanical and does not need a compressor. However, as was mentioned earlier, the load need to be centered and the unit needs to be properly sized for the load.

Albert porter uses a Vibraplane under his SP10 MK3 to great effect. You may want to PM him. I also use a Vibraplane for my suspended SME 30/12. The key with Vibraplane is to preload it as it is most effective if operating near its max. design load. The improvement for both my SME 10 and 30/12 was dramatic.

You might want to consider getting a large enough surface on either device so that you can also put the motor controller up on isolation. That too matters.

There is a recent review in which the Minus K was directly compared to a Vibraplane, and the guy preferred the Minus K, FWIW. Whichever device you decide to use, the supporting rack must be very solid and stable.

Excellent guidance and insights from Pryso. Always a pleasure to see coherent thinking. It's more effective to thoroughly understand the problem(s) you're trying to address and select solutions which address them than to try stuff willy-nilly... or to do nothing.

I could not afford a Minus K, but I did want to isolate my 90 lb., unsuspended TT from floor-borne vibrations, particularly as the floor is suspended wood (though well built and sturdily framed). In trying relatively inexpesive solutions, one finding was consistent and clear: the closer the isolation element was to the cartridge/tonearm, the more deleterious side effects were audible.

High mass, unsuspended tables are rightly prized for their massive dynamics and good low frequency response. Light weight, springy tables simply can't compete in this area because movements of the cartridge and arm induced by groove modulations can actually wag the table. High mass tables resist this, but will be susceptible to floor-borne vibrations, including those from the earth itself, passing traffic, etc.

Placing isolation devices immediately below the table did lower the sound floor, as expected, but it also diminished dynamics and bass response. In effect, I'd converted my unsuspended table to a suspended (if massive) one.

What to do? Increase the suspended mass by moving the isolation devices from beneath the table to beneath the stand that supports the table. My TT sits on a rack that, together with all the other equipment on it, masses over 500 lbs. Isolating this entire mass from the floor would address the floor-borne, earth-borne vibrations while allowing the high mass of the TT (plus 400 lbs of additional mass) to provide the solidity it was designed to offer.

Being relatively poor, I opted for a poor man's well engineered solution: Sorbothane pucks. Before you snear (as I once did) consider that I was NOT placing these in contact with the TT or anywhere near it. I tried that for laughs using Vibrapods and it sounded dreadful. OTOH, placing Sorbothane pucks of a durometer and number appropriate to the mass being suspended beneath the 8 feet of the entire rack made a nice improvement. Backgrounds were blacker and low level details more audible with no detectable penalty in dynamics or bass response. A win!

As a bonus, isolating the equipment rack from the floor helped isolate all the other gear on it too. Fewer audibly obvious benefits for a CDP or amp than for a super-sensitive phono pickup, but certainly no harm.

So... keep your high mass TT as rigidly connected to whatever supports it as possible. Isolate from the floor, yes!, but do so as far away from the table as feasible. Isolation requires movement and movement saps dynamics.

My $.02.

Consider for a moment that there are various vibration isolation designs. Of the mass on spring type, there can be vertical isolation, a combination of vertical and horizontal isolation and a combination of vertical, horizontal and rotational. There are three rotational directions of isolation - roll, rock and twist. No, I'm not talking about the Peppermint Twist. The definition of isolation in a particular direction is the ease of motion in that direction, thus good isolation in the horizontal direction would be characterized by being able to push the component easily in the horizontal directions. For turntables, which exhibit high rotational forces around the vertical axis, especially when heavy platters are involved, I suspect that isolation in the direction around the vertical axis is not desired, since rotational forces produced by the rotating platter would put the isolating system into oscillation, impacting the accuracy of the speed of the platter rotation. For that reason, I would constrain any isolation device so that only vertical and horizontal directions are addressed, keeping the rotational direction around the V axis stiff. If I'm not mistaken I believe that's exactly what Minus K did - constrain motion around the vertical axis by eliminating the isolation capability in that direction.

Excellent guidance, particularly from Pryso.
My 50 kg non-suspended soapstone plinth TT sits on 4 Audioquest sorbothane Big Feet, these act as shock absorbers.
This very heavy, in some extent suspended TT sits on plywood/aluminium plate that sits on 3 metal spikes to DIY metal rack filled with sand. The total weight of this system with all equipment on rack is more than 150 kg. Can´t be moved nor rocked by hand. This metal/sand stand has 4 spikes to floor.
I´m happy with the situation but I will replace the spiked plate with sandbox. I think this would make further progress. If I´m wrong please enlighten me.

While it's fairly accurate that heavy rigid support structures cannot be "moved or rocked by hand," the problem arises in the way seismic vibrations manifest themselves - by moving the entire building, not only up and down and side to side but also in the rotational directions. Hel-loo! For this reason even very rigid and heavy support systems can be easily moved by the seismic type vibration. As I mentioned earlier the effectiveness of isolation is actually a function of the ease with which the component can be moved in the direction of interest. Counterintuitive, eh?