Personal experience tells me there are so many factors that addressing the weakest link makes the most difference. Narrowed to preamp/amp again the weakest of the two being replaced gives the most benefit. The following can be a litmus test as to what factor one might see as the most important/critical in a system. For each of you I ask, what piece of equipment would you be most resistant to replace and what piece would you least like to see replaced?
For me a recently acquired preamp has made the most difference. Once the change was made the shortcomings thought apparent in my amp largely disappeared and the strengths of the amp became more apparent.