If you want to test how a surround (non-Atmos) receiver behaves with a Dolby Digital+/Atmos signal, you can download the sample files from:
Downloadable Dolby Atmos Trailers - Dolby
But make sure that it's sent to the receiver as a pass thru / bitstream, i.e. the player (Blu ray with USB or PC, etc.) connected to the AVR via HDMI is not doing the decoding.
I don't know what the Firestick 4K does when you play an Atmos track on Tidal.
But I know the behavior with the Apple TV 4K (I already posted some of this earlier, so some info will be repeated):
(1) With Atmos enabled on the Apple TV, it will 'repackage' the Dolby Digital+/Atmos to Atmos NAT.
This is the same with different streaming apps (Tidal, Netflix) on Apple TV 4K -- so the signal will appear on the receiver simply as "Atmos".
Other streaming devices (example Roku) stream the original Dolby Digital+/Atmos to the receiver, so the signal appears as Dolby Digital+/Atmos on the receiver.
(2) If I turn off the Atmos setting on the Apple TV, it will say "this device does not support Atmos", but it still sends a discreet multichannel track to the receiver, either as:
a. Dolby Digital (not Dolby Digital+) - again, the Apple is re-packaging the original Dolby Digital+ signal)
b. Or multi-channel PCM
depending on the "Change Format" = On or Off setting. I don't know if you lose the audio from height channels, or if the Apple TV is downmixing them to the 5.1 channels.
In all cases above (1), (2) a/b, it plays the same source file from the Tidal (actually Amazon) servers -- I confirmed this by looking at the network traffic.
#2 Above is with the Apple TV 4K still connected to an Atmos receiver.
But I believe a surround (non-Atmos) receiver will behave the same way, mainly because of the fact that it says "this device does not support Atmos" but it still sends a discreet multi-channel signal. But I cannot say 100% for sure, someone needs to test this.