i vote for amp. plenty of good 2ndhand deals come and go, all you need is time, but youre not in immediate need of a better bass. 5 months ago i got a warwick corvette proline from 1993 for 550 euros. 2 months ago i got a Trace 400W tube head with 4x10 for 650 euros. Soundwise both were an upgrade. Amp is very important for good sound and even for definition. in my band, i noticed i got far better definition after i got the trace. it really helped. I say: get a nice head, youll be happy with it. If i have GAS, its always for a bass. But soundwise, amp is as important as the bass, and imo even MORE important than the bass.