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I use Herbie's Iso-Cups under my tubed poweramps and Tenderfeet (is 'TenderFoots' the plural of the brandname 'TenderFoot'?) under my CDPs and preamp. They sound plenty fine to me, but I'm no GEA who hears (or thinks he hears) all sorts of subtle differences. I judge these things in the long term and remove them if I think their presence is decreasing sound quality. My equipment rack is high-mass, with 2" maple shelves and 1"-diameter verticals, and is spiked to the concrete floor. See it in my System .
Far as I'm concerned the principal use for sorbothane type isolation product (or any other soft materiels, depending of the frequencies you need to damp) is the need for your component to be isolated by frequencies thru the floors or component supports. This is (IMHO) the exact opposite of the results/requirements of hard, pointy, devices which seem (are alleged) to improve resonance issues by providing a path for vibration drain from the component.
FWIW, all my hard pointy products are used as balast to keep my component chest well anchored to the floor when the system is 'rocking'. :-)
I still have Navcom Silencers, Mod Squad Soft Shoes, and Vibrapods. I use them occasionally, usually in combination with the brass cones. My rack and speakers sit on brass cones. Sometimes I'll place the SRID's between a component and shelf. I can't say that they dramatically improve performance, but I own them, and they are not worth selling on AudiogoN, AND they will not scratch the equipment that I set on them like some brass cones can.
I like to think that even though I have the latest Audio Points cones and stuff, I can still kick it old school!
I use some sorbothane footers under a tubed power supply. I also use Rollerblocks, Aurios Pros and a Neuance platform under various tubed components.
A friend and I recently tried sorbothane footers under his tubed CD player and preamp. They made a nice change. The music sounded more natural. Previously he was using no vibration control.
I have 23 McCormack "Mod Squad" feet in my system, mostly under equipment or one support platform. I have another 12 or so Iso-bearings (soft ball design in Delrin cup) that support my Aesthetix Io.
No hard feet under anything except turntable stand, tape recorder stand and built in speaker spikes of the Dali Megaline.
If you look at the upgrade Grand Prix amp stands, their best footer is carbon fiber discs sitting on the floor, connected to ball socket design foot.
it is my experience that hard objects "enhance" resolution, while soft objects "attenuate" energy in the upper mid/lower treble region. yes, this is a generalization and i can mention a few exceptions, such as dh cones and dh squares.
i prefer soft objects such as sound fusion sound boosters, furniture foam and memory foam.
i also have foam cable lifters manufactured by westlake audio.
I use Herbies Iso-cups in conjuntion with Gabon Ebony balls under my transport and preamp.
My Hydra and Dac rest on thick Maple blocks with different composite footers.
A lot depends on what component, weight, type of shelf and or material they're on.
Experimenting and proper compliance (soft, medium, hard) is the key factor in determining my sound.
Many years ago I bought into the hype around Black Diamond Racing cones/pucks. I invested quite a i bit in their products which now sit abandoned on a shelf in my listening room closet. With as musical as my system should have sounded, there was a hardness to the upper midrange and lower treble using the BDR products. That's objectionable on its face but worse yet, it made for a disjointed sound and soundspace. (I'm not intending to bash BDR products as I don't doubt they can be effective in specific systems)
Then...I discovered Herbie's Audio Lab and their Tenderfeet. With both regular hardness tenderfeet and extra firm versions for very heavy components, I realized a linear upper midrange/lower treble response and a coherent soundspace. To this day all of my components are damped with tenderfeet.
I also use Herbie's decoupling gliders and bases to decouple my speakers and subwoofers from the room. Smoother sound and tight deep bass that is not transmitted through the rest of the house as before.
The rest of the family really likes those devices!
Not all soft rubbery materials are alike. The viscoelastic soft rubbery materials can work very well in constrained layer damping applications, the rest of them probably do more harm than good. When in doubt decouple. One advantage I can think of for rubber footers is they won't scratch the floor. 😀
I may be mistaken, but I believe lots of normal people just make do with the stock rubber feet components come fitted with ;-). It has been argued that a component "voiced" with it's stock rubber feet won't sound as it's designer intended with other feet, if in fact those after-market feet change the sound.
The thing about rubber feet is their ability to isolate is not linear, but rather frequency-related. Garden-variety rubber is pretty effective down to maybe 10Hz, the frequency at which it's isolation-capabilities are in sharp decline. That is why rubber (including Sorbothane, though it is available in a range of durometers) is known to cause soft-sounding, smeared bass.