The best upgrade is a balanced one, preferably the one that addresses the weakest links in your system. Short of that, it's like asking the question : Do you prefer to listen to a signal from a superb source that has been compromised by downstream components (including AC hash and room acoustics) or do you prefer to listen to the output of superb speakers that has been compromised by upstream components (and AC hash and room acoustics)? If the system is already fairly well balanced and you want to take it to the next level, I would agree with Warrenh: improve the source first. After that, it would depend on what the new and higher resolution uncovers. Experiment. If a weakest link is revealed, address it. If all the remaining elements are equally "weak" relative to the new source, upgrade the speakers next. There are no easy universal answers, unless you win the lottery...but then again...
