Room. The room is usually the hardest to change. The room will dictate speaker and listener placement.
Once that is determined, a savvy buyer will consider speakers (including subwoofers) that will perform optimally in the chosen room. Sound dispersion, wall, floor and ceiling reflections and absorption treatments, bass traps, subwoofer system and room loading, digital room correction will all be considered.
Once the speaker selection is pared down to those that will complement the room, the buyer will select speakers that sound "good" to him/her (or maybe look good). Once that is settled, a good audio dealer will be able to hook you up with appropriate amplification for the chosen speakers. Source should be considered last, as it is the easiest to change.
Note: it is amazing how even inexpensive speakers can sound fantastic when pulled away from the walls and reflective surfaces and into the middle of the room. I have a couple of Harbeth P3 speakers that are easy to move (I can't leave them in the middle of the room, so I put them back when I am done listening). In that nearfield position, with me on a chair away from the back wall, It is very hard to imagine anything that could sound much better (well, actually I can, but tube amplified horn-loaded compression drivers with a custom crossover or Wilson Chronosonics driven by D'Agostino M-400s is fairly out of my budget - and wouldn't fit in my room anyhow)!