Believe what you wish to believe. They physics don't necessarily play out.

Several physics items are possibly at play:

1. Isolation pads COULD isolate speaker cabinet vibration from being transferred into the surface that they are sitting on, which could then in turn vibrate itself and create unwanted low end sound which would compete with that coming from the monitor speaker itself. Unless you have some flimsy panels on the desk somewhere, the likelihood of this is rather small, particularly if you are listening at low levels.

2. The isolation pads CAN make the monitors less efficient when the monitor itself is not fixed well to 'ground', in that when the woofer cone pushes against the air, it doesn't have the benefit of a solid connection, rather a more springy connection to 'ground' to push against except it's own mass, not it's mass and the mass of the earth. This is why you see the exact opposite of monitor isolation pads being sold in the hi-fi speaker market - metal inverted spikes designed to be placed under hi-fi speakers and 'pierce' through carpeting and pad down to a more solid foundation. I'm talking about items like this: http://www.oregondv.com/spikes.htm

However, with the weight of each of your speakers, which you said was 28 pounds each, there's likely enough mass there to not notice a big issue with low end punch if you did mount them on iso-pads.

You will likely have more of an issue with room acoustic cavity modes getting excited by your monitors, as well as comb-filtering that could result from having the speakers too close to reflecting surfaces.

The first issue is primarily caused by the geometry of the room, and where you place your monitors relative to the geometry of the room and the rooms natural frequencies. This article: http://ethanwiner.com/acoustics.html would be a great place to start to try to get an understanding of what the cavity resonances, also known as standing waves, are and why you should be concerned about them - Spend more effort working this out, than worrying about isolation pads. It's a much bigger issue than whether your monitors are on iso-pads or not, because it doesn't matter at what volumes you are listening, the standing waves in the room WILL get activated at nearly any playback level and will deceive you at your listening position.

Pay particular attention to Figure 11 and the discussion around it - highly important stuff there. Also down near the bottom of the article, maybe 3/4 down is a discussion of a tool for free download called ModeCalc - this will take the dimensions of your room and show you specific frequencies that are likely to be issues in your room, where you place monitors, etc. The physics are pretty simple, and this tool will yield quite accurate results unless your room has non-parallel walls, many large items breaking up the geometry of the room like bar-counters, big furniture, etc.

Also pay attention to the side-bar in the article about creating a reflection free zone. The advice in that section is quite good, and it's going to help you avoid comb-filtering. Key advice in that section for most home studio design is adding absorption to surfaces that could reflect mid and high frequencies from the monitors to the listening position. One trick not mentioned in that article is that you can do this by putting the absorption on any wall (and ceiling) where if a mirror was placed in the same position, you can see the monitors in the reflection of the mirror. This is ray-trace acoustics and spending the little bit of money that you would on monitor iso pads, on absorbing material like rigid fiberglass insulation, is going to be much better spent for most home studios which are often bedroom spaces, with rather narrow wall dimensions. I'm a bit lucky in my studio, I have 2' from my monitors to the wall behind the monitors, and 4' to each of the side walls - where the reflections off of those surfaces back to the listening position is significantly longer than the direct path from the speakers to my listening position.

That article is probably the most concise and honest article on home listening room and home studio acoustics that you can find. While the author has a company that sells acoustic treatment (RealTraps), he also offers up a host of practical and rather inexpensive acoustic treatments in this article that can be purchased mostly at home improvement stores and installed with a little bit of effort.

You just bought some nice monitors - make sure they play nice in your space. Iso-pads would be very low on the list, if at all, for getting the most out of your monitors. Most of the endorsees of these iso-pad type products have already gone through the rigor of having a treated space or had a ground-up purpose designed control/listening room built where many of the common issues of home studios can be avoided altogether.

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