Conventional wisdom says speakers. However, whether buying used or new, there are many speaker choices out there to be had in the $1-2k range that are awfully good. Worthy electronics (amps, etc) cost more, whether new or vintage. I spent 3.33 times more for a used amp than I did for new speakers, including new dual subwoofers. It's a nice system, with the best SQ I've ever owned.
I have many times heard a pair of speakers sound much better after an amplification (pre, power, integrated, whatever) upgrade. Amps don't all sound alike to me.