The problem with the poll question is, it entirely depends on where you're coming from, what your perspective is. What I mean is, if you're using a mid-level receiver then you add a good amp (and still use the receiver as a pre/pro), you'll notice a difference. I was using an Onkyo TX-SR702 receiver, added a very good budget amp (the Emotiva LPA-1), and noticed a WORLD of difference, huge improvement. Then I yanked the Onkyo and installed the Emotiva LMC-1 pre/pro (cheapest pre/pro I know of, very low on bells & whistles, but considered to have excellent sound quality for its low price so I'd expect it to sound better than the Onkyo) -- but honestly, keeping in mind it took me over an hour to make the switch, I couldn't hear any real difference.
So the answer entirely depends on your system, room, speakers, etc. If you have a world class system except you're using a cheap receiver as a pre/pro, obviously changing to an excellent pre/pro will make a relatively
huge difference that you will hear. If you've got a budget system then you add a really nice amp to it, you're likely to hear a bigger difference than if you simply put in a good pre/pro but kept a crappy amp, speakers, etc.
I think the answer for you will depend on your circumstances and the question has no "correct" answer. If your room is acoustically decent, you have good speakers, then you add a good amp where before you were powering everything with your receiver, chances are the amp will have made a huge difference. If you have a world class system in a treated room with outstanding speakers and a very good pre/pro and amp(s), then you start switching out amps and/or pre/pros, who knows, maybe then you'll hear bigger differences between good
pre/pros than you do between good
I would have voted for choice D: "It depends."