yes there are reflections and yes a tilt back can help

yes there are vibrations and a mechanical DE-coupling from the floor may help

getting a speaker higher might just be about getting it closer to you

but generally when a speaker is lifted from the floor ... OR moved further away from a wall (boundary)
it is to acousticaly De-couple the speaker from the boundry

when a speaker is coupled to one or more boudaries there will be an enhancment of the lower frequencies

the distance required is relative to wavelenght
the lowered the frequency the longer the distance

this could get complicated
so
it might be better to have a google surf and chase a few of the key words seen above

these principles are seen in
Soffit Mounted Speakers
Horn Loaded Speakers
PZM microphones

To add to Kev's insightful post -

It's not uncommon to place a large, inert, dense trapping material below said amp to help minimize the LF reinforcement.

For example, getting the amp off the floor by 2' then placing a 4" sheet of Owens Corning 705 below it.

This done in an already acoustically-enhanced environment will really let the amp shine through.

That being said, you may just prefer the sound of the amp with that boundry boost. Otherwise, it may sound too thin.

I've had my amp-a McIntosh MC2100- on the floor for many years. I tried placing it on a stand about 2 feet off the ground, but I could never hear a difference. Maybe I'll try it again...

Mooney....
McIntoshes are computers silly. You're thinking of Mackie Amplifiers. mNow THOSE are nice amps... :-O
(jk)

I've had my amp-a McIntosh MC2100- on the floor for many years. I tried placing it on a stand about 2 feet off the ground, but I could never hear a difference. Maybe I'll try it again...

I'd be really worried if your $12,000 amp sounded different when on the floor. :mrgreen:

It was a mere $500 when I bought it, and I just looked at a used McIntosh site and they have one at $875.00 (as of 1/30/08). Maybe it's got a bad hard drive...

Good medicine guys, thank you.

I'll certainly expand my research as well as record some clips using a variation of the techniques thrown out here to get some first hand ideas of what will happen. It was on the agenda anyway, I just thought it would be better to have an educated approach first before just lifting these heavy buggers up.

/Shack

Are casters considered to be adequate for decoupling from the floor?

Also, I was raised to always put the tube amp heads on the floor instead of on the cabinets to help prevent microphonic tubes.

Amp off of head .....yes

Casters decouple.....maybe. Depends on the floor material and the casters and their mechanical nature. Fenders.....no. Bottom tray of a high-end road case....yes.

Thank you Davedog

i'm assuming the original question was about guitar or bass amps?

Anyone who's ever played in a band or mixed bands live knows that guitar amps tend to easily get too loud too soon. Sometimes it's ego, sometimes it's simply accidental. (Think about it: If the guitarist is standing up, he/she is then about four or five feet ABOVE the actual sound coming out - at floor level. Feel's good on the back of the thighs, but quite far away from their ears.) This, IMHO, is one of the biggest reasons why amateur bands sound so bad....no one is really listening to each other, but moreover trying to get loud enough to be heard.

A friend of mine fixed this kind of problem with a really loud, rude band by simply putting all of the guitar amps up on road cases, behind their shoulders/heads. Suddenly, everyone started playing better (hearing themselves better too), and the resultant stage level dropped a good bit, too, making the FOH mix all the better.

Go look at any picture of Springsteen's band live in concert. All the amps (usually Fender) are titled way back, almost too far. They are pointing UP at the musicians. (Bruce's amp has ALWAYS been very very loud onstage, but again, it's pointing UP, at him, not under his feet and out into the crowd.)

It makes for better ensemble playing overall. (You don't hear the sound of a violin or viola coming out four or five feet away from the player, right? It's right next to their HEAD, actually.)

It's an old trick, and it works quite well, live onstage, or in the studio.

Head off cab....cool, good to know. I can try that too. Though, I do have some combo amp, but it'll be nice to experiment and hear the differences.

Joe - yes, guitar amp, sorry. I'm with you on the playing live issue, but we often have people (from the crowd) join us on stage so putting my amp UP on anything always scares me. I use in-ears anyway...life is much better that way.

Thanks everyone.