Thanks to everyone who posted. Sunday night, after a lot of web surfing, I put in an order for Enduro 6901 Abec-5. Per veganbikes' comments, QBP's catalogue is no longer available to retail customers, and I visited the websites of BTI, Merry Sales Co. and Euro-Asia Imports, just to see what the options were, going through an LBS. BTI was the only one of the three that had extensive choices of Enduro bearings, but they only had Abec-3, did not have Abec-5. They had Phil Wood too. Merry Sales had very limited choices. Euro-Asia Imports had Phil Wood but not 6901 (which is the type I need), didn't have Enduro at all. I purchased the Enduro Abec-5s from Universal Cycles, four for $47 shipped. They shipped them Monday, due to arrive today.

I will be curious to see what was originally installed in my Pro+ hub. I put 20,500 miles on it which, apparently, is a lot better than some have gotten out of their PowerTap hubs. It might be because I am fairly light (150 lbs), don't ride in the wet, and only raced for one and a half seasons.

UPDATE, 12/21: I got the bearings. On the exterior of the races, they are marked "61901 SRS ABEC5 CN". 61901 and 6901 are the same. I don't know what 'CN' means. The key thing is that there are indeed two levels of workmanship offered by Enduro: The Abec-3 bearings are cheaper, about $5 each plus shipping. These Abec-5s are $10 each plus shipping.