I think he was probably suggesting putting a combo amp on a chair instead of on the floor. This minimizes the amount of bass that is transferred on to the floor which then resonates into the room. It simply eliminates some potentially unwanted bomminess.

It's more a matter of trying to reduce the sound from the amp that is reflected off the floor and into the microphone. Some people suggest tilting the amplifier upwards, but this misses the point that it is the sound at the mic that is the problem. I usually place an amp on something like a hi-fi loudspeaker stand with a thick carpet or rug on the floor underneath.

Don't forget that the whole of the room acoustics come into play when miking up a cabinet, not just the floor reflections.

He is talking about decoupling the cabinet from the floor. The same principle is true with any perpendicular surface around a cabinet or sound producing device. Sometimes you will want the additional boost to the particular frequencies effected by this. An increase in lower mids and bass frequencies is the general rule and these will be damped by the type of material the floor is constructed of. The problem is a general 'slewing' of these frequencies resulting in a smeared response and lack of clarity. If you do your research you'll find most quality rooms use a Mo-Pad or something of this nature under guitar cabinets. Its all about the decoupling. search that term and you'll find a new world of guitar cabinet/amp clarity from just adhereing to basic physics principles. Its not just mic selection and preamps and electronics that make a clear sound. Its always the environment first.

In addition to what was already said, a couple other things to consider.

Mic placement (distance), angle relative to speaker, the polar pattern of the mic, itself...and amplifier volume.

A mic with a good narrower pattern will reject more noise from the back and sides, which means it will help minimize reflections off of nearby surfaces, and should greatly reduce reflections from more distant surfaces. But, that depends on how close it is to the speaker. The closer it is, the more direct sound it will pick up, relative to any unwanted reflections. Even if it does pick up some, it may be minimal. Move it further away, and there is more chance of unwanted reflections getting into it.

The angle of the mic, relative to the speaker, (on-axis, etc.) also has an effect, on both direct sound pickup, and relative amount of any reflections that may be present. Obviously, the more a mic is turned sideways, even near the grill cloth, the more it may allow reflections to bounce back into the polar pattern to record.

The issue of the volume of an amp should be obvious. The louder it is, the more chance it has to project a strong level at another surface, and the stronger the reflections will be. A lower level has more chance to, first, not even reach another surface at enough volume to bounce back much, and, second, if it does, it will dissipate more quickly before bouncing all the way back to the mic.

Using a good quality cardioid (or hypercardoid) mic close up, pointed directly at a speaker, at a height off the floor to minimize coupling and nearby reflections, and at a moderate volume, will make "room treatment" less of an issue.

But, that may not be the sound you want. Pretty much any other mic'ing method demands a better room to minimize unwanted anomalies.

Just some extra little tidbits.

Kapt.Krunch

If the amplifier cabinet is open in the back, you might want to try putting a microphone on the backside of the speaker. And then you may also want to invert the phase of that microphone. This can give you a completely different perspective. But that's not always possible since I don't know too many Marshall amplifiers that are open on the backside of the cabinet. And use a SM57. I've never had much of any problems when I use one of those regardless of where they speaker/cabinet has been placed. You might even want to try a low-cost ribbon microphone? Cascades or some such. I simply recommend this because I don't like to reorganize where guitarists want to place their amplifiers/cabinets. In a Marshall stack that as 4 speakers in it, choose the speakers that are in the top part of the Cabinet as opposed to the ones near the floor. So everything that's said and recommended is true and not true. You don't know until you try. It's like asking whether it's better to take your car key and stick it into the ignition lock with or without a key ring. Folks with a heavy key ring of keys can damage your ignition lock. And I've only had a problem with that once. If you saw my key ring you would say OMG! You have so many keys! It's because I have so many locks. So is it better to keep losing your keys or not losing your keys? That is the question.

In fact sometimes you get a warmer tone out of a guitar amplifier if it is on the floor due to the coupling to the floor. So it's only bad when it's not appropriate sounding. You don't know until you try. And that's why God created equalizers.

It's all good.
Mx. Remy Ann David

RemyRAD, post: 391818 wrote: And that's why God created equalizers.

Sounds just like what the government does, Remy!

God created the problems with frequencies...and then had to create equalizers to fix the problems he created! And then, the equalizers are known to cause even more problems. So, create more solutions to solve the problems of the solution to the initial problem...which introduces even more problems! So....duh

Kapt.Krunch

Yup, it's actually amazing what fine recordings one can make without any equalizers at all. You go through numerous different microphones with numerous different placements and positioning, first. They equalizers are utilized then, to shape or slightly modify the sound. They really aren't there to try and fix what's not right to begin with. That's like trying to put another Band-Aid over top of a bloodsoaked Band-Aid and then all you get is a big mess. As we all know, once the Band-Aid is soaked with blood, you remove it, you clean up the wound and you start with a fresh Band-Aid.

I've got blisters on my fingers!
Mx. Remy Ann David

To add to what Remy said - Here is a small clip from a real old song I did the Lead Gtr part on and the engineer used 5 mics at different distances including one in the back + direct in. It was a 5amp studio amp with no effect pedals. Recorded old school straight on through suerarick.us/music/Pusher Guitar.mp3