For $1500 you could get an excellent used preamp and power amp combination that would be a good base to upgrade the speakers later. You could always try to borrow some gear to see if you notice the improvement with your current speakers.
Since you mention tubes, you could pick up a something like a Conrad Johnson PV-10 preamp for around $600 and get a nice solid state amplifier and some good cables to link them together with the rest. Possible candidates for an amp would be a Conrad Johnson MF2100, or MF2300, or a McCormack DNA 0.5 or 1.0.