This comes from the recollections of Saturday Evening Post reporter Jules Siegel, who was following Dylan's last few tour dates and was present in Dylan's Vancouver hotel room/suite when Albert Grossman arrived with the acetates.  Quotes from Siegel about the episode appear both in Clinton Heylin's book The Act You've Known For All These Years (p.11) and in an interview with Siegel by Derek Barker, published in the UK Dylan fanzine ISIS, Issue 78 (p.28).  See Appendix F for detailed references.