Definition of the 'Reference Axis' and sonic effects of on/off axis listening
To answer the question: if you look at any pictures I've posted of my measurement set-up (I'll look out another) you will see that the mic is (generally) face-on to the tweeter, that is, perpendicular to the baffle and at or about aligned with the tweeter dome. That's what I call 'the reference axis'. The speakers is designed, adjusted and auditioned in that way *with the grille on*.
It is a fact that off-axis all (conventional) sound sources, including woofers and tweeters, have a progressively diminishing audio output with increasing frequency. So what happens when you listen off axis - that is, the speakers facing forward along the room and you sitting some degrees one side or another? First, listening laterally off-axis,the top end of the woofer/midrange's operating range will have diminsihed in loudness. Second, the top end of the tweeter's operating range will have diminished in loudness. You may or may not prefer that sound, but measurably there will be a small energy dip somewhere around crossover frequency because a speaker can only be optimised for a (flat) response at one point in space - the reference axis. If you sit at that point, you will reproduce the design conditions. If you don't sit at the reference point, you are not replicating the design conditions, athough you may well prefer that sound. It is entirely a matter of taste and how your ear/room/music/personal preference guides you as to what is the 'right' bass/mid/top balance.
As to adjusting toe-in and the quality/quantity of bass seeming to alter: I doubt that this is a physical phenomena because over quite a wide positional arc the measurement microphone shows no difference in the bass response. Much more likely in my opinion is your perception of the bass is changing because as you are listening off-axis the altered intergration between the bass/mid unit and the tweeter is fooling your ears into thinking the bass has changes. It's not likely at all as the wavelengths of bass notes are about the same dimensions of your room. Hence swivelling the speakers a few degrees is unlikely to make any measurable difference.
Adjusting the proximity of the speakers to any nearby suface - floor, wall, ceiling - and/or the construction/damping of those surfaces will produce a likely sonic effect and one that can be easily measured.
Alan A. Shaw
Harbeth Audio UK