Wattage 4 or 8 ohms?

There isn't a universal answer. In my case, as per the manufacturer, the power recommendations are based on the standard 8 Ohm power rating for amplifiers, but, the manufacturer assumes that one is using a high quality ss amp that "doubles down" and suggests that if one is using a tube amp they choose one(s) with increased power output to compensate for the minimal impedance load. For example if the speaker has a 4 Ohm nominal/4 Ohm minimum impedance and the suggested power requiremnets are 50-250 Watts per channel, choose an amp that can provide between 100-500 Watts per channel into 4 Ohms. In most cases that would mean a high quality ss amp that "doubles down" rated at between 50-250 Watts per channel, or a tube amp rated at between 100-500 Watts per channel. FWIW, there is an old audiophile rule of thumb that suggests that one start with doubling the manufacturers minimum power recommendation.

I drive my Thiel CS6's with a Pass Labs X350. I has a power rating at 4 ohms impedance of 700 w/ch. My speakers have a listed max power rating of 500 watts, but when talking to the manufacturer, they were ok with that.

It's all about quality and quantity.8^)

Tonywinsc, Jim Thiel has told me himself that (at least with his older models) that his power recommendations are based on standard 8 Ohm amp ratings, and the assumption that the customer is choosing high quality ss amps that are capable of "doubling down". With your CS6's minimum impedance of 2.4 Ohms, I suspect your Thiels could handle well over your available 700 Watts into 4 Ohms. With that said, I'm sure your system sounds fine.

Hello,
What to do if I have speakes with 6 Ohm impedance, but amp has only 4 or 8 ohms output options? What should I choose?

Thanks

Dmitry....

You may detect a sonic difference between the two taps, depending on the design of the speakers and your ears/expectations. You should try both taps and simply pick the one that sounds best to you playing the music you enjoy hearing. You won't damage anything by doing so.

Sometimes folks find that the 4 ohm taps can produce a more solid bass, some that the 8 ohm taps can produce better highs, but it all depends on how your speakers impedence curve matchs the amps output impedence. Speaker and amp impedence values are not fixed at anything as absolute as 4 ohms and 8 ohms. Those a just nominal numbers.