Which parts suppliers are the most helpful and reliable?

Perhaps Parts Connexion expects one to know what they want, specifically(of course; I have no idea, what you were asking). Like Newark and Mouser, most of the phone-people, with which I’ve spoken, don’t seem(too terribly) electronically savvy. But; I’ve never had an issue, in my extensive dealings with them. Parts will generally arrive, within one or two days of ordering. Michael Percy has been another reliable, and- much more informative, source. Especially, for some of the more esoteric components/materials I’ve needed(though; somewhat harder to contact, from time to time). He’ll even match some components, to within a 1% tolerance, for a slight uptick in price. https://www.percyaudio.com/Catalog.pdf

IMHO, you have an unrealistic expectations of a parts supplier.  Their job is to supply and get you parts quickly.  Technical specification can be found on the part's manufacture's website.  If you want advice, I would suggest one of the many audio/electronics DIY forums. 

My inquiries to Parts Connexion related to a component built by them and supplied to another customer. I wanted to commission something similar. I'm looking for parts suppliers who are more than just a middle man between manufacturers and consumers, suppliers who cherrypick their offerings for their performance and reliability and who are  knowledgeable about components to which they pertain -- audiophile parts suppliers, not supermarket sellers. Conscientious ones.

Oh.   So- you mean, like Michael Percy.       Here's how to contact him:   https://www.percyaudio.com/Catalog.pdf

Depending on needs, I’ve ordered from Michael Percy, Parts Connexion, Newark Element, Mouser and others. I’ve never had an issue, but you need to know what you want when you place your order. My most recent purchase was from Newark -