Inverted phase preamps?

Amp end or speaker end - it makes no difference.

Lots of recordings are made out of phase and you can hear it. I got some new Cream remasters which have songs out of phase and I'll play with some switching later tonight.

Tvad. Probably a bit of an exaggeration saying most manufacturers. Good to see you are reading :)
Having the phase invert on the pre is certainly easier. I can't say I use it a lot on my pre thou, except for a couple of LP's. Generally I notice it the most is on big vocals when the singer is breathing in when he should be breathing out. The Elvis 5LP box set live in Vegas is my most obvious example of the music out of phase.

Both my amp & preamp invert phase, does this mean I do nothing?

Yes, it brings it back to an even number so no need to do anything.