Microphonic Amps?


I have a valve amp (EL34 in ultralinear config).
I have read that valves are microphonic...what does this mean exactly. Will outboard vibrations be picked up (why?) and transmitted as sound deficiencies (again how?) ?
If this is the case, do i need to take equal care isolating my amp as much as I would, say, a turntable?

thanx for any help
128x128gaffer74
Thanx for that everyone (nicely explained :-) )
In that case, do I take as much care as with turntable placement....would an anti-resonance base help or would tube rings be a better bet re: next purchase?

PS, my amp is integrated, with the pre-amp section being 4 small GEC valves, and the power amp section being 4 large Svetlana EL34 valves:
http://www.affordablevalvecompany.co.uk/
Well, you'll get no answer to your question that everyone will agree on. For small tubes in my amps I have found it easiest, and most effective, to buy tested low noise tubes in the first place on which I place those inexpensive rubber damping rings. Then I try to reduce exposure from floorborne vibrations by using a combination (sandwich) of materiels with different resonance frequencies to absorb the vibrations (a DIY project). Some times I use cones (or not) to couple the component depending on the effect it makes. I do not try to couple the rack itself to the floor (except for stability) as I feel the combo of materiels used under the component are sufficient, besides I have wood floors and I think coupling might just enhance floorborne vibrations effects on the components. JMHO. Good luck getting any consensus.
Gaffer74...Think about it...a phono pickup is designed expressly to pick up small vibrations, whereas tubes are designed to minimize microphonics.

Turntable vibration control is essential although IMHO it calls for common sense more than expensive equipment. Amp isolation is something you can play around with when all the significant problems of your system are solved.
Thanx everyone.
My system currently sounds fantastic, and I cannot really find significant probs...but would just like to optimize anything that can be (if that makes any sense).
I have my system in a particularly bass heavy corner of the room on a steel glass stand with halved squash balls under each foot to try to minimize vibrations further.
Again..I can't hear any odd/problem areas as such, but just wanted to get the best in the area I have the equipment currently.
Thanx for all your input....I think I'll spend further dosh on music instead ;-) as I feel I have it isolated as much as i can, and it sounds great.