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.
Which parts suppliers are the most helpful and reliable?
I found VH Audio the best to deal with in my limited experience. Chris Ven Haus has always been willing to discuss issues and reliably advise. Part Connexion is another story. They neither seem anxious to sell me parts or advise me about components, in fact, I received no response from them over the course of two weeks of respectful queries. What about Sonicraft and Hifi Collective? Others? I am especially interested in hearing about suppliers with willing and reliable counsel.