Originally Posted by: dlwalkeI'm just trying to understand first, in what ways do amps differ tonally and how much of that does does not ultimately come down to frequency response characteristics that can be adjusted with tone controls or an EQ pedal.[/quote]

This is potentially a heavy discussion. There are many resources on the web about this. Here's a decent one that goes into some detail.

https://www.guitarplayer.com/technique/all-about-classic-tube-amp-circuits

In short, the differences between the general categories of guitar amps are due to a few interconnected factors.

1. Pre-amp tubes & how they are used in the circuit.

2. Power tubes & how they are used in the circuit.

3. Type of design (class A, B, AB, etc.)

4. Topology of the circuit design.

And that's even before you get to speakers & enclosures.

So, the answer to your question here is that is a great deal that affects the basic tone beyond the EQ controls that cannot be adjusted to make say a Fender Twin sound just like a Marshall JCM 800.

Also, the term frequency response can refer to the amp signal output or the speakers.

Originally Posted by: dlwalkeSecondly, I'm curious which of these characteristics can modelling amps successfully approximate.[/quote]

Depends on who you ask. :)

In my experience, modeling amps can & do successfully model a wide variety of amps, effects, speakers, microphones & acoustic environments (halls or rooms).

There is a case to be made that "the real thing" sounds different. But quite a few blind tests have been done to demonstrate that modelers are quite good at modeling.

The only difference I've noticed is the playing experience. I can & have dialed my Line 6 POD in to simulate playing through a Boss SD-1 into my cranked Marshall JCM800 half stack. The resultant recorded sounds are virtually indistinguishable from each other.

The thing that is extremely different is the playing experience.

It feels very different to play through the POD than the Marshall. When I say feels, I mean physically, in a tactile way how each system responds to my fingers/hands motions. And of course even if I turn up my PA or studio speakers with the POD blasting, it's not the same physical playing experience as playing in the room through my amp.

Originally Posted by: dlwalkeSo I think I'm on safe ground by assuming that the frequency characteristics of different amps often ARE differernt and I would think that this COULD, at least in principle, be manipulated with tone controls or EQ pedals/software. That is, maybe amp A is more boomier than amp B but you could increase the bass knob and maybe decrease the treble knob on amp B to bring it into the same frequency realm as Amp A.[/quote]

I think you would have a very difficult time doing that. Based on my personal experience I would be tempted to say, no, you can't. But I leave that thought experiment to a more knowledgable electrical engineer. I know a few amp designers. I think I'll ask them when I get a chance.

But more to the point: why? If you want the sound of a Fender Twin, get a Twin. If you want the sound of a Marshall then why would you buy a Twin & then buy boxes & effects in an attempt to make it sound like a Marshall?

Originally Posted by: dlwalkeDo different cabinet/housing designs (e.g., closed vs open back) affect amps in ways other than simple frequency response.[/quote]

Very much so. I'm assuming at this point you mean the freq response of the amp.

Originally Posted by: dlwalkeI have seen amps or software that let's you select different cabinets. But the amp itself has a particular housing (e.g., open vs closed back) so how successful can an open backed modelling amp be at replicating the sound of a closed back amp. If the difference is just that one is more bassier than the other, I would think that this could be modelled by adjusting the EQ. But maybe it is more, or other, than that.[/quote]

Modeling amps are fundamentally about software emulating hardware. So, there might be differences once you get to the hardware stage. But a great deal of modeling units are just software. So, it's kind of apples & oranges to make that comparison.

Consider also that there can be a considerable difference between the sound of any given amp as you hear it standing in the room playing through it & the recorded sound of you playing through the amp. And in some applications all that matters is the end result as a recorded guitar part sounds coming out of a stereo system. Or of a guitar mixed in with the rest of a band all coming through a PA system.

All that will either match or not someone's preconceived experience or expectation of what an amp is "supposed to sound like".

Originally Posted by: dlwalkeThen there is, I would guess, the dynamics of the response of one amp compared to another. That is, as the gain or input to one amp increases, maybe it starts to compress and saturate sooner than another.

You bet! And then you have to factor in the technique, touch & dynamics of the guitar player! It's complicated. :)[br][br]

[quote=dlwalke]I have no idea how modelling amps do what they do

There are many great sources on the web for this topic. Examples:

https://en.wikipedia.org/wiki/Amplifier_modeling

https://www.premierguitar.com/articles/Amp_Modeling_Reality_Check

[quote=dlwalke]Speaker size. Well, if a given modelling amp has a single 8" speaker, I would think it is fundamentally incapable of accurately simulating a Fender Twin Reverb for example with it's 2 12" speakers.

To my knowledge most modelers that have this speaker option are only software.

[quote=dlwalke]Is moving a lot of air something that you only get with big speakers driven by a high watt amp?

Yes. You can plug a modeler into an amp or PA & turn it up. You can use a modeling amp that has enough power to play loudly & get the same result. Either way, moving a lot of air refers to loud volume.

[quote=dlwalke]Will a high wattage amp with a big speaker(s) move a lot of air even at a low volume?

No, low volume equals moving only a little air. :)

[quote=dlwalke]

I have heard people talk about amps with a tight, focused bottom end vs others that are more flubby (I think that's the term I heard). What contributes to this? It doesn't sound appealing. Does anyone like a flubby bottom or is that just the mark of a lower quality, prersumably less expensive, speaker?

[p]It has to do with how the speaker responds to playing through a cranked amp. It can be desirable in some cases. Depends on what kind of audio result you want!