Do not underestimate the importance of custom tuning to your ears

Many audiophiles will reject a high end speaker even very expensive ones all because it was too bright, too harsh, not enough bass, imaging problems etc. 

What we must remember is that many of these rejects may in actual fact be great speakers if only they were retuned to the persons ears. 

Custom tuning the crossover can change the brightness, the tweeter level, image depth, and more.

It would not be right to reject a speaker design that is fundamentally correct apart from the way its tuned. 

Some speakers are fundamentally wrong and no amount of retuning will fix them. These are the real rejects and we must not lump good speakers in with these. 

The difficulty is that its not easy to tell which speakers can be retuned and which ones cannot. 

All of this applies when buying a speaker for the first time or upgrading. Many speakers end up being sold all because of retuning issues and room acoustics. 

Custom tuning to your ears is the key. 

That’s not an answer to my question.  Try to answer my question based on your premises.  

What source do I deem reference and begin tuning from there when each source presents a different sonic portrait?  

If your speakers are "out of tune" it wont matter what source or cables you use. It will never sound right. That is why audiophiles who try to blame their cables and source for poor sound quality can never achieve the sound quality they want. 
You use any or all of the sources and tracks you like to help you tune the speakers. Once its done, you will hear the differences in quality between sources and tracks much more clearly than before.

"Do not underestimate the importance of custom tuning to your ears"

...silly reading the topic, I made the assumption that kenjit needed an ear tuning and was ready to strongly suggest the over-the-counter earwax product Debrox.  With no affiliation, it IS a recommend, just the same.  More Peace, Pin  
Wouldn’t disagree with your initial argument. I think many audiophiles lack the knowledge and skills to make these mods in speakers as they stand today.
I have been thinking about incorporating an L-pad attenuator the way Infinity and other manufacturers used too to help fine tune sound in my speakers. They have become much less common.  I have been trying to find a good looking, high quality part to incorporate.  
I was just discussing this with a retailer I am having my products listed at and another option is to simply sit with the customer to do the final tuning to their taste.  Especially at some of the higher price points.  Customers should be allowed to get their speaker their way.  Would radically decrease buyers remorse.  

One interesting approach is what Michael Green does.

Michael believes in tuning everything and is big into modding gear. He is a very good guy and the folks in the tuneland forum are very smart in this space.