Which Do Ye Prefer . . .

DVD instruction or 'book learning' for your opening / defence ?

It's probably best to actually play the game and figure it out as you go

Grab an iron and an ironing board and some clothing and start ironing until you learn how.
I don't think that's the best way to learn.
Spread the knowledge already learned from pasted mistakes, so that others will not have to repeat the same experience.
If they don't believe, they will learn the hard way by playing a million games and still not reach mastery.
Unless we get corrected for our errors, we will continue to do them, until we realize or get told how to fix the errors.

So reading or viewing all depends on if you need to know that subject matter and if you prefer one over the other.
I found it is always best for me to have either two different books or videos covering the same subject to really understand the material. Some times it starts by a short video or preview and then I get the book.

For most things, practice makes perfect. Chess should be no exception.

no. Practicing the right the makes perfect. Practicing the wrong way or the wrong thing will make you play badly perfectly

books or go home

I use a database and an engine and use books as reference materials to understand why certain moves are common if its not clear to me.

databases and engines are mean

@ezasucanget actually, the best players in any sport play a lot, that's why they are good. The main (practically only) difference between practitioners of any skill is how much they do! (Time )

The lion‘s share is practicing. A DVD or a book gives you some hints, but that‘s just for the first steps of a long path.

By the way, regarding „theoretical sources“: database and tree is very important