6 ohm speakers with 4/8 ohm amplifier

It's about damping factor. The 4 ohm tap will increase the damping factor, most speakers are designed for this, better control over the woofer. However, you may not like it so as suggested try them both and see what you prefer.

Like it was suggested in the first post try both and let your ears decide which is the better of the two. No harm to your amp.

Maybe I should ask differently.

If a speaker manufacturer has the following specs; do they mean the recommended power in 8 ohms or 6 ohms?

Specs:
Impedance: 6 ohms
Recommended power: Minimum 50wpc