As if we don't have enough on the plate already, let me add another course anyway, Speaker Toe-in.
First let me say that the way the soundstage presents itself is one my most highly regarded qualities in sound reproduction to me. A really convincing soundstage is what separates mid-fi from hi-fi
You could have a great FR and perfect ambiance, but if the soundstage is lacking, I am unimpressed.
That said, I have been placing special attention to it in the last few days and critiquing it. My first impression was that I loved everything happening between the speakers, but I thought I could improve what was happening beyond each. So I increased the speaker to speaker distance by 2". May not seem like much, but there was a noticeable difference. While the soundstage was a bit wider, it sounded a bit like it was curving.
While this is a inaccurate simplification, it represents sorta my impression of the shape of the soundstage.
So I decided to change the toe-in. That is to say, have less of it. And now my impression is more like this.
FYI, Where 0 degrees represents the speaker pointed directly at your ear, before my toe-out was about 4 degrees. And now it is about 12 degrees.
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A few thoughts on toe-in in general. Most reading has you toe-in so that the focal point is a foot or two behind your head. Also described as having the speaker point directly a few inches to each side of each ear. This is the configuration I have had mostly up to this point.
I am bringing this up partly just to see if anyone else has done experimentation in this area. I have seen other peoples setups vary from the speakers pointing directly at you to the speakers facing directly forward. Obviously, the speakers and room play a major role in what works best here. But my early findings suggest that excessive toe-in, that being pointing close to directly at you, while helping the middle in some cases, seems to make the soundstage outside the speakers seem a bit unnatural to me.
In any case, I am interested in any stories any of you may have on the topic.
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As far as how this relates to the thread topic, one will find that the high end roll off is generally greater as you toe-out. This may happen as well in the area where the mid/tweeter XO occurs for the midrange may roll off a bit more towards its upper range. This will be VERY speaker dependant, or more precisely, driver dependent. It is also true that some drivers are a bit flatter a bit off axis. So some may see a flatter response due to greater toe-out. Again, very speaker/driver dependent.
But for those who feel your system a little bright, a little more toe-out may help you.