4 8 or 16 ohms.

I believe it is 4 ohm. 

 

The 4 ohm input impedance characteristic was chosen for two principal reasons. First, the B&W engineers felt that most good amplifiers (with which this system will, no doubt, be used) would deliver their maximum power into 4 ohms or less. The second refers to the ideal situation where the driving amplifier runs out of volts and current at the same time, thereby delivering the maximum power to the speaker load. This, according to Dibb, is more likely to occur with a 4 ohm load.

Copied from a Stereophile write up. 

https://www.stereophile.com/content/bampw-matrix-800-loudspeaker-page-2

 

Someone said to me it does not matter what ohms I connect my speakers to. He said just use the ones (ohms) that sound best.

                    'Someone' is correct (you won't  damage anything).

                                        Enjoy your new amps!

https://www.stereophile.com/content/bampw-matrix-800-loudspeaker-specifications

 

B&W Matrix 800 loudspeaker Specifications

 

 

 

Sidebar 1: Specifications

Description: Three-way, reflex-loaded loudspeaker with line-level bass equalizer. Drive-units: two 12" polymer-cone bass, two 5" Kevlar-cone midrange, one 1.25" ferrofluid-cooled, metal-dome tweeter. Frequency response: 23Hz–20kHz, ±2dB free-field. Impedance: 4 ohms nominal (3 ohms minimum). Sensitivity: 93dB for 2.83V at 1m. Recommended power: 150W–800W.

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Use the 4 OHM tap.

Your friend is correct in the sense that it will not hurt the amp or speakers if you try/uuse the 8 or 16 ohm taps.

However, the frequency response of the crossover will be slightly altered from the factory’s intended response.

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https://guitar.com/features/opinion-analysis/what-is-impedance-guitar-amplifier/

https://forum.audiogon.com/discussions/trying-different-output-impedance-taps-with-tube-amps

https://forum.audiogon.com/discussions/any-experience-using-8-ohm-taps-for-16-ohm-speakers

https://forum.audiogon.com/discussions/16-ohm-speakers-any-amp-sounds-better-with-more-resolution-speaker-ca