Hi Steve,
Your experiences are not uncommon and there would appear to be something in this. Like yourself I use a combination of wood & tubular steel while acknowledging that glass is the material of choice in many popular racks - especially the thicker gauge stuff. There are those who decry tubular steel as conferring an "unnatural" sound when in conjunction with glass (Mana Acoustics, and Russ Andrews are 2 examples) but consider the case of the time tested Rega Planar turntables which used glass close to the most critical interface in a hifi system, the cartridge (by using glass platters) and ended up with a highly acclaimed product (!)
Like yourself, having noticed the effects, many years ago I tried a few simple experiments involving dampers and supports, oddly enough using an amplifier not dissimilar to yours - Naim Pre/Power/PSU. The most susceptible items were the Naim amps that employed extruded casework. I repeated one of these Trials as a Blind Test for the benefit of the Forum. This involved putting a simple damper on top of a CD player to illustrate that some form of microphony exists. The Blind Test - conducted with an impartial listener, was 100% successful. (You should be aware that Official Blind Tests need 1000s of participants and a statistical success rate of >80% - a bit like our AVR Cable Blind Test, which is going to use 6 people 
) Now this does not suggest that any modern components are microphonic, but it is scientific fact that conductors, such as PCB traces or wirebonds, or indeed non-microphonic components, could vibrate at AF in the same way as the electrodes in a valve, but within a complex soup of magnetic/electrostatic fields, or perish the thought , the Earths (mind you not all casework is steel). Once you factor in specific resonances and tuned frequencies such as those presented by glass, steel & rubber and the rationalisations (and the "sound-effects") do start to look/sound more interesting?
Personally, I'm satisfied that my own experiences, and those of the Blind Test are sufficient reason to believe that there may be better mechanical synergy possible within a given system. Especially if an analogue turntable is involved, then appropriate support is more of an imperative than an option.
Hope this is helpful.
Kind regards, Bill.
