How to "beautify" (soundwise, of course) this old Pioneer amp?

[nonexpert]

Hello!

I'm new here, and already I'd like to ask a few questions...

Well, I've got this old Pioneer stereo from my grandpa which is called "Stereo Music Centre M-6500", and it actually sounds better than I thought, but I was wondering if it is possible to enhance this beauty... Would it make sense to exchange the transistors in the amp assy? Or the two (biggest) capacitors? What part(s) is/are most crucial for the sound? Or is this useless because the design of the whole device probably isn't worth working on?

Another question: while the speaker output is quite enjoyable, the headphone out is pretty bad. There is no dedicated headphone amp, but two resistors to handle the current for the phones. I have no experience regarding such things, so I'll ask another rather stupid question: could there be a big sound difference if one exchanges these old resistors with modern, matching ones?

Guys, I hope I didn't waste your time with this... If you reach this point, thanks for reading!