New AMP downgraded to S1 flashes white and orange

Have you waited 10-15 minutes after you did the downgrade? The white/orange led means the device is updating:

https://support.sonos.com/en-us/article/sonos-led-lights

It is long after the downgrade was completed and the AMP was associated with my S1 system, I just tried again with Spotify as source it lasted 5 minutes before it failed, flashing white and orange, the temperature of the unit is just hand warm appx 28 degere Celcius = 82F. With S2 firmare it does not happen.

What speakers are connected to the Amp? Do you know the impedance of the speakers?

It could be a load issue that is only noticeable with the S1 app. Have a look at this page for some troubleshooting steps;

https://support.sonos.com/en-us/article/no-sound-from-amp-with-a-flashing-orange-and-white-light

The problem also happens without speakers attached, on S1 firmware, I have tried 2 different set of speakers:

1) B.I.C Ventura Formula 6, 4ohm

2) Argon 6340, 4-6ohm, with new hi-fi speaker cables 

Yesterday evening I reverted to S2 firmware and left the AMP play for 12 hours with the Argon speakers, in the morning it was still playing, at lunch time I downgraded to S1 again:

1) I perfomed a factory reset according to Sonos specification 

2) I used the Sonos app to perform the downgrad according to Sonos specification 

3) I closed the Sonos app

4) I opened the Sonos S1 app and searched for new device according to Sonos specification 

5) I started to stream the same radio program from DR LYD that was playing all night without problems on S2, 2 minutes later the AMP stopped playing and started to flash white and orange.

In my opinion there is a bug in the S1 code.

I will try to call Sonos support.

Today I spend 44 minutes on the phone with Sonos support, they decided to replace my AMP due to hardware failure.

On top of what I had tried we also tried with a different power cable.

I am not convinced that this is a hardware failure, I still think that it is odd that the AMP did not fail with S2 firmware.

I will get back with an update when I have recieved the replacement AMP.

My replacement AMP has been recieved today, so far it works fine, when l looked in “Om mit System” (About my System) I can see a difference:

Faulty AMP

Version: 11.15 (udgave 572259130)

Replacement AMP

Version 11.15.1 (udgave 572261162)

As long as it is disconnected, the faulty Amp will eventually disappear, as your network updates IP leases. You might try rebooting your router, if this disturbs you, but leaving it alone is fine. 

Am having exactly this problem, though it took me long enough to make the S1 downgrade work that I have not tried upgrading to S2 and then downgrading again to S1. Music plays on two different Amps with S1 for maybe 10 minutes (even on very low volume) and then stops and goes to blinking orange and white. S1 software version says 11.15.2. 
 

I have spent two sessions with support, unplugging, no speakers, one speaker, etc., not getting anywhere. Agree with Ottens_ DK that it feels like S1 software problem. Neither Amp gets even warm, never mind hot. Both Amps replaced ZP100s that had been in place for 15 years with no problems, which implies to me that speakers and cables are fine. Did the replacement to simplify two zones where there are TVs. 
 

Replacement amp still working fine for Ottens_DK?

And any new ideas?

Never mind.  On third call to support, concluded that the Amps are faulty and need to be replaced.