I have two different Nakamichi BX-300 cassette decks. One was made in
1984, and the other one was made in 1986.

While I had the 1984 deck opened up to do some repair work, I opened
the 1986 BX-300 to take a look at a part number. Looking inside the
1986 deck, I immediately noticed an "extra" PC board mounted on the
right side near the back. According to the service manual, this
"extra" PC board is the Input Amp.

The 1984 deck does not have this Input Amp PC board inside it. The
service manual verifies that it was not installed in the 1984 deck.

MY QUESTIONS:

Why was this board added?

What benefit does it have?

Are later BX-300 decks that have this board "better" in any way than
earlier versions?

These may be dumb questions, I know, but curiosity has gotten the
better of me.