An anechoic chamber will also provide s smooth response, but I dont particularly care for he way music or even speach sounds in such a room.
Anechoic chambers provide flat response (no room gain), which is why speakers are measured in such a room. Let's not continue to conflate "flat" with "smooth".
However, playing around with the speaker toe-in ... ie Speaker positioning will also yield quite dramatic changes
Sure, but we were discussing placement. It is a stretch to refer to toe-in as making use of placement, since the speakers stay in the same place as you rotate them towards or away from you.
Toe-in can affect the smoothness of frequency response IF your speakers have poor off-axis response. This is because you never hear a speaker alone, you always hear a speaker and its reflections. So even when the speaker has flat on-axis response and is pointed towards you, the side of the speaker is pointed towards the side walls. Reflections coming off the sides will sound very different from the direct sound from the speaker. When they combine, it will result in very uneven response.
Below is an example of a speaker with poor off-axis response. Notice that as you move off-axis, the high frequencies don't just roll off, they really change in character.
By comparison, speakers from Kef or anything from one of the Harman lines (Infinity, JBL, Revel) will have very consistent off-axis response. Side wall reflections from these speakers will sound very similar to the direct sound from the speaker. When they combine, the sound of the speaker won't change.
Below is an example of a speaker with decent off-axis response. Naturally, the higher frequencies are going to roll off, but it looks fairly similar to the on-axis response.
Toe-in will affect the frequency response smoothness of the top speaker, but not have much effect on the frequency response smoothness of the bottom speaker.
But here Are other measures (besides speaker placement ) that can help significantly, such as diffusers or absorbers at the First Reflection point & behind the listening position, especially if its close to the rear wall.
If I'm using speakers with poor off-axis response, then I'll absorb the side wall reflections (so the sound of the speaker won't change in character with a ragged response). If I'm using speakers with good off-axis response, then I'll leave the reflections alone (so I get a wider soundstage).
In both cases, I try to cover the wall behind the speakers with as much absorbtion as I can. This has nothing to do with smooth frequency response but more to do with improving imaging in the front soundstage. All I want to hear from between my front speakers is their direct sound, not extraneous reflections that will muddy the delicate phantom imaging. Same reason to put absorbtion on at least the middle half of the wall behind me (since the speakers are pointing there and could result in distracting reflectons).
