I believe everyone in the Velvet Underground endorsed Vox amps during the time of White Light/White Heat (if memory serves it was recorded 1967, released january 1968). More than likely it was a cranked Vox amp of some sort. If you're really interested, my friend recommended this book, which, by the way, neither me or my friend are in any way affiliated with:
http://www.voxguidebook.com/