OP
Both. A lesser preamp can limit otherwise stellar amps, and a great preamp can help a bit (but not fix) a lesser amplifier. I've recently upgraded my preamp (Aric Audio Motherlode XL), and 2 amplifiers (CODA S5.5 and Aric Audio Transcend "Push Pull".
As individual components each FAR exceeded my expectations, but driving the S5.5 and Transcend with the Motherlode XL was much more than the sum of the parts.....more of a 1 + 1 + 1 = 5 type scenario, which I've never been able to experience in my own systems.