8ohm speakers 4ohm amp power conversion

Hi,
I am about to purchase some outdoor patio speakers. Kicker KB6000w, they are 75w per speaker. I was wondering what is the minimum amount of power I can get away with running them at? Say if I buy an old amp with 50w per channel. Will that run them ok or will it affect the sound quality. Also if I buy a power amp at 4ohm, 75w per channel, how much power do I lose trying to run the 8ohm speakers on it. Sorry if this a silly question, not really an area of expertise for me. TIA

 

You can calculate for it, just follow the ohms law. P = U * U / R

75 watt in 8 ohm == SQRT ( 75 * 8 ) == 24.5 Volt.
24.5 * 24.5 / 4 == 150 Watt

But some 8 ohm speaker can always be conneted to a 4 ohm amp as long as the power does not exceed the speaker.
A 4 ohm speaker to an 8 ohm amp could ruin your amp if is has no protection in it.
But most amplifiers allow 4 ohms speakers..

Any speaker which is overloaded will burn out, tweeters first.

So, 50 watt into 4 ohm gives an amp with an voltage at the terminals of 14 Volt.
Which makes 25 Watt into your 8 Ohm speakers.
75 Watt into 4 ohm ends up into 8 Ohm at 75watts/2 almost 38 Watt.

You can only match your 8 ohm speaker for 4 ohm amp with some audio-grade transformer.
I don't know if the still exist these days, and for that wattage.