I had a similar problem with YUDEN000T02 media burned at 4x. The first several 4x burns worked fine (good quality scans). Once the 1655 burned enough of these to consider it "learned", it turned off SB and failed to burn this media every time. I sent back that drive as defective. It wasn't until my 2nd 1655 did the same time that I dug into this further to find out that SB off was to blame. Nobody ever explained WHY the first few 4x burns with SB on were good and then afterwards its learned strategy for the same media was so bad. So, I changed to burning all TY media at 8x, and I also turned SB on for known media. Now I find my 1655 burns are about the same as the 1640.
I would think BenQ probably got back a bunch of drives because they don't clearly explain the mysterious behavior of SB with learned media at certain speeds. I read everything on their site, and it was only here that I found the answer. It's almost like all BenQ drives should come with a note "in order to learn how to use your drive properly, please go to Club.CDFreaks.com".