Helping friends/relatives with system purchases, your experience.

The most difficult but most productive part for me is setting aside my own preferences and prejudices. Without that I'd tend to recommend what I would buy for myself with whatever money they have to spend.

I wouldn't.  Not only that sound is very subjective, but also you might be engaged forever or even blamed when something goes wrong, or when they hear better sounding system in friends home (what a shame  :) ).  It is a little bit like recommending stocks to friends - it won't end well.  My friend asked me once for advice.  I spent time looking for best possible solution, but he didn't listen and did it his way.

I generally don't get involved...   

It's useless . You need to suffer a good result yourself - go to the goal, conduct tests, spend money ... These people (even if the result is good) will not appreciate it - it got too easy ...