One thing I have noticed in my own singing and in others is that when we listen for squillo the mind somehow knows that thicker chords make greater buzz. This is a natural thing from our middle and speaking voice. For the tenor this is a big problem because as we ascend through the passaggio if we our mental image is built on intense squillo and we monitor from an auditory perspective, which, face it, is the most readily accessible to the mind, we may end up increasing the level of chest participation as we ascend in order to secure the ringing quality of the voice.
This, my tenor friends, is a serious problem. Increasing chest to get resonance means that we will destabilize the top voice because we are basically creating a heightened antagonism with the muscles that would stretch the chords.
May I suggest something? Let others listen for squillo in your top, and you focus A LOT on a sense of velvety dark flow (1st layer) in your voice in the top. This doesn’t mean that we don’t listen for the squillo at all… we do need to tune the right harmonics, but what I am saying is that much of this is a mind game.
Those who don’t have experience singing with squillo in different acoustic spaces, and that pretty much sing in rehearsal rooms, and at home cannot really understand the depth and the importance of what I am talking about. I am talking big stage here. If you can’t hear your squillo enough, it will be almost immediate for you to increase the level of chest in order to thicken the chords and get more buzz. You have to fight that urge!! You must, or your voice will wiggle, the vibrato will become bad, or worse the mind will sense something wrong with this chest mechanism (though you may not actually perceive that consciously) so that mechanism will stop working and you will flip into falsetto on a high note… not good.
The solution here is that you should aim to perceive the buzz up front… a vibrator sensation or air spinning up front in the maxillary and frontal sinuses or even in front of your face outside… but as far as sound goes, think dark velvety as your main focus. Even if you hear squillo, ABSOLUTELY CONVINCE YOURSELF to not listen to it except to recognize it is pitched correctly. Listen for the velvet up top and you will be far better off. The sense of flow will increase and the voice will actually stabilize and the squillo become more intense.