Because I know how tenors sometimes drive ideas into the ground, let me make one thing clear about cavita’, or 3rd layer of sound, as I refer to it. There are schools, like the Melocchi school that advocate “massima apertura della gola” – greatest opening of the throat. Clearly, these methods will require more pharyngeal space, which involves keeping the tongue forward in the throat to a large extent, and elongating the resonance tube by lowering the larynx.
As I have tried to convey, the 3rd layer or cavita’ is not the whole voice, it is one aspect. Just like any other aspect of the voice, too much is like putting too much salt on your food – you have to avoid it because it tastes awful. I know there are probably some who will take the idea of tongue forward in the throat and exaggerate the cavita’. This is not right. Even the audio examples I posted, from Bjoerling to Corelli, are examples of possibilities. It doesn’t mean the singer always sings with maximum cavita’ – well Corelli probably most of all did, but that made his voice more of one color.
The tongue should never block the space of the throat. However, to orient the tongue always very forward in the throat means to not sing any vowel properly. Remember we are talking about in the throat because as far as the mouth goes, the tongue should stay forward always, and increasingly so as we ascend.
There is a balance, and that balance is dictated by the sound (unless you can invent a machine that singer’s can afford where you can observe the inside of your throat while you sing… that would be a cool bio-feedback tool). The correct sound is not constricted, but rather flows. So the tongue cannot constrict the pharynx too much, but will rather gravitate toward being as far as possible from the back wall of the pharynx without emphasizing too much one aspect of the voice. Sometimes, you will want less EU, and more EH to make the sound brighter. Let me refer you to perhaps one the most important documents in tenor studies – a video of Jussi Bjoerling singing live “Salut demeure chaste et pure“. This is one of the most important case studies of vocal technique.
Notice how he sings “chaste” with cavita’, in the EU position. He sings most of the aria right on the balance between cavita’ and brightness (this means that the 1st formant is balanced with the higher ones).
You should notice, that he NEVER constricts the throat, by putting the tongue back on the pharynx. He always has a healthy head tone imbedded in the sound (1st layer). The voice flows. This is the one of the greatest recorded examples of tenor “flow phonation” to be found. It is as close to perfect technique (incorporating all sorts of varieties of technical possibilities) as one can hope to find.