Big topic!
We have tried lots of isolation options in the last year - mostly low cost and easy to remove - in my room and system, and reached some conclusions. There are lots of threads on this and related topics for other views.
FWIW and with apologies for the length...
1. We hear hi-fis and rooms, not just hi-fis. Which boxes matters and what they sit on can make a difference, but do the room shape, the floor, the furnishings, the ceiling, how carefully you are listening to what and at what volume, what you expect to hear, the state of your mains supply and the local EM and perhaps Wifi environments, plus how tired you are. The list is endless and often not easy to translate from one room/ system/ person to another.
2. I have a bouncy floor, lots of Naim boxes and an LP12 (on the wall), and I am liberal with volume. For me, my ultra-heavy granite rack is VG - I am confident that a Fraim or equivalent would do less well, given my floor. I have tried boxes on floor (audibly bad), boxes on cheap Sound Org racks (apparently fine), and boxes on my rack (definitely better). Quite separately, I have also heard the same boxes in several different rooms.
3. Isolation options that we tried under Naim boxes included Naim cups-and-balls and clever rubbery feet plus wood, MDF, 2 sorts of glass and more granite.
On older 'olive' Naim boxes with big transformers, we all agreed that putting HRS Nimbus Assemblies (i.e. feet) between box and granite shelf gave a small uplift at higher volume. So did putting a hefty rubber/ metal weight on the same boxes - but even smaller IMHO. We heard no repeatable uplift doing the same with newer Naim boxes or Lingo PS. Everything else we tried was worse or made no difference in multiple sessions (as blind as I could do) with multiple listeners.
4. In another room, with a different shelf and different makes of electronics, the 'best' answer will presumably be different. Few with a firm floor and a good rack report gains from DIY or bought isolation extras (though not none) unless they are pretty purist and the system wildly expensive. However, quite a few seem to use butcher's blocks or isolation platforms with less purist racks and sideboards. It is possible that they are all deluded of course, but it seems unlikely.
5. Very few of us seem to have tried more than a few options in any room, so there is no exhaustive and easy to interpret database on this. That is a major problem when looking at isolation because: -
A. If the upgrades are not illusory, they will be small - unless something was far from ideal before (e.g. an old LP12 on a sideboard or something rattles).
B. It is easy to fool yourself and hard to do a useful blind test, and 'different' is not the same as 'better'.
C. Like cables, there is a lot more apparent science than actual science on show here, but also plenty of vigorous opinion.
D. Plugging and unplugging cleans connections, so the new version may well sound better.
E. You may have moved boxes so that you now have (say) big transformer and preamp/ source further apart, which may well sound better.
F. You may have taken a bit more care on cable-dressing than usual this time - it can matter.
6. You'll probably find more agreement on turntables. Suspended designs like LP12s are said to work best on a 'light and rigid' support with good reason, and extra layers of compliance (beyond what you need to stop the needle bouncing) usually make sound quality worse.
Most modern TTs are not suspended and are more forgiving than an LP12 of what is under them - they may be good with a fairly compliant/ bouncy support and/ or just sitting on top of a decent rigid rack. However, the best isolation for any non-massive TTs is very often a wall shelf - not because the top of a rack necessarily sounds worse (although it often does if being really picky) but because of doors slamming or heavy feet or buses passing or whatever.
7. CD players may fall into the same category as valve kit for different reasons - more sensitive than solid state to good isolation, but rarely dramatic. My old Naim CDS2 certainly sounds better on HRS feet on granite shelf that it does sitting directly on the shelf, and granite beats the old Sound Org and that beats the floor.
8. Speakers are toughest and I haven't even started experimentation yet. Without knowing more than I do about the most relevant modes of vibration and all the resonance numbers (and speed of sound in all the materials and so on) I know that I can't analyse what is going on, but I am instinctively suspicious of many claims of a universal upgrade to speakers in general. What energy is supposedly going where?
Take an easy question - why are my speakers on spikes?
I had Shahinian Compasses for years and liked that the company line was that the little rubber feet were best, no WOO needed. Then I heard spiked granite platforms for them (resting on £1 coins) and that was clearly better - mostly tighter bass and better stereo image.
My B&W 804s came with spikes or rubbery feet, so we put on the spikes. In all my fiddling about with isolation, I have yet to put on the rubber feet to see if (wth a vastly heavier speaker in a different place on the floor) that is better or worse today. I will try, honest!
After trying that, I could try Isoacoustics gadgets or cheaper rivals, but I would probably start with the sort of firm foam pads you see under studio monitors. However, with so little understanding of what we are trying to achieve or how, I suspect it would be a case of 'shooting in the dark'.
Which speakers on what floors have people found to benefit from which of the after-market isolation products? Which were rubbish?
Even those who aren't firm believers in 'The Wisdom Of Crowds' may be interested...