Jobst Brandt recommends preloading the adjustable bearings. How much and how do you measure it? Only by feeling? I always set my bearings to roll as free as possible without play. Seems I was wrong. Any opinions?

Bollocks,
Free with no play is the way

Free with no play on BB, and solid wheel axles. With QR axles, you need to have the very slightest play because the action of closing the QR tightens the preload on the bearings. Practise on your wheels. If you still have play after fitting them to the bike and doing up the QR, they were too loose.

With headsets, the test is more whether there is "indexing" or a notchiness when the handlebars are turned, or whether there is movement felt when you apply the front brake. To test the latter, put on the brake, wrap your free hand around the top headset cup and rock the bike back and forth... if there is looseness, you will feel it in that hand. Ideal adjustment is in between those two tactile measurements.

Jobst talks from a very engineering perspective, and IIRC, his preload suggestions aim to get the balls or rollers doing what they are supposed to do from the get-go. But it does become a balancing act, and to me, accurately measuring preload is more trouble that it's worth.

+1

I actually get the hubs as close as I can, then between hub maintenance cycles, I actually adjust the QR tension +/- 1/16 of a turn to get that perfect balance. I'm always making sure the QR is tight enough, but you do have some wiggle room in how tight it is.

When in doubt, make them a little tight, as they break in, they can only get loose.

In any properly functioning bearing assembly, all the bearing contact the cone/race at all times, distributing the load and shock loads evenly. If the assembly is loose, then all the load is taken by one or a few bearings which concentrates the forces and leads to premature wear.

-1

It's easy enough to make them just right, but too tight is worse than too loose. "Break in" for overtightened bearings is just premature wear.

+1 to this