I was going to point that out as well, but I figured if the artist is smart he would just use some "transparent" interface pre-amps (which all studios should have). Sure, different interface pre-amps also sound a bit different, but not anywhere close to the difference two non-stereo pair SM7B's are. I also doubt that he would want to lunk around on a heavy ass 1073

 

Tell him thats a beginner mistake. Its simple. The vocals need to fit the song. Unless he is using the same kik drum, the same bass, same synth patch etc in every song, it will be a different set of values for each song. Just make the vocal feel right per tune. In the end they will sound consistent organically. Do stem mastering, this will give you the ability to smooth out any spikes which can only really be done when the album sequence is set. Have fun with it and it will come through in the mix.

 

I would say that the key to getting them all the same when recorded at different studios will be a two step process, broadly speaking.
Step 1: You will have different levels of room noise to remove. So on each recording I would analyze an area of the recording with no vocals to obtain the background noise. I use FL Studio's Edison to remove noise after analyzing. There are many ways to do this, I'm sure you have your own preference. After removing the noise analyze the frequency spectrum in something like ProQ. I would pull up two of the recordings from different studios and compare them, each in their own eq. This will be especially important if the microphones vary from studio to studio. This will allow you to make adjustments to get a base line started. If you have an example vocal from the artist you can load this into Tonal Balance Control as a reference track. This will allow you to put Tonal Balance Control on the new vocals, pull up the curve from the reference, then make adjustments to an eq (placed before Tonal Balance Control in this chain). Once you make these adjustments check again against the reference until you have a satisfying result.
Step 2: Build your vocal chain and when satisfied save this chain and apply it to all future recordings, after completing step 1.

Step 1 is the key here as it is basically a "clean up" to get the input into your vocal chain prepared in such a way that the vocal chain affects all of the recordings the same way.

 

That's why i suggested EQ matching first.
It will solve 90% work.

 

Well, I said that half-joking because as said by others this is a complex task for a pro level. Room/mic can change more than frequencies. Dynamics and all that stuff.
That being said, it would help.

 

This is the way.
Of 40 years ago. Guy will never listen.

 

I want to thank everybody for the useful suggestions, they helped me a lot!
Cheers

 

Nectar, Satin, The Oven, 3k eq boost 2-4db up for extra clarity. BOOM