Does this happen only at low volumes?

If yes, and assuming the Connect is on variable volume setting, what happens if you keep its volume slider at around 80% and moderate the sound levels via the Rotel volume control?

The Rotel is RMB1512 digital amp, there is no volume control on it.  By 80% volume level on the Sonos connect side I assume you mean adjust the volume limit to 80%?

Yes, but since the Rotel has no volume control being a power amplifier, 80% will be too loud. Is it possible that there is some signal auto sensing and going to standby happening with the Rotel at low signal levels?

PS: what happens if the auto turn off/signal sense switch at the back of the Rotel is set to off?

Moved to off position, will report back

Kumar, you may have done it!  No music cutting out after hours of play.  Hope this stays, thanks!

Do you remember what position the three position switch was in earlier? Signal sensing or 12V trigger?

I ask because the downside of the solution is that you now have to turn the amp on/off yourself every time...

It was on “signal sense”, I turned it to “off”

A possible way to get the benefit of signal sensing minus the attendant problem you were seeing - the Rotel has an input gain adjustment. That may allow a setting that lets the Connect volume level slider be set to a high enough minimum volume level that does not cause signal sensing to have the Rotel go into standby mode during quiet passages of music. If that works, the three way switch can permanently go back to the signal sense position and allow the amp go automatically to standby mode only when there is no music playing.

Worth a shot, the necessary experimenting. If it does not work, turn signal sensing off again and do the Rotel on/off manually.

Hello,

I have had the same problem with this Rotel 1512 amp, and I thought I post here in case other people had the same problem. Kumar’s answer above is correct, but it does not mention how to adjust the amp sensitivity. On the user manual on page 3, there’s a diagram that shows the 6 adjusting screws on the front panel under the amp fault led lights labeled as “3” and explained on page 8 as “Input level Controls”. You need a small screw driver to adjust these. Basically you need to lower the amp amplification by turning the screws counterclockwise and increase your input volume. Basically by turning amp screws to the left, your 80% volume won’t be as loud as before but it’s still the same amount of volts coming in which makes the amp stay active. 
 

In my case, I had to increase the Denon 3700H receiver speaker levels by 9dbs each and turn the adjusting screws of the Rotel a quarter clock-wise from their off position. It doesn’t go into sleep anymore.

If not going into sleep mode is acceptable, can that not be achieved by just toggling the signal sense switch to OFF? What the OP has done, accepting the need to do power cycling of the Rotel manually whenever needed as the trade-off.

No because then your amp uses more power even when you do not use it. It should not go to sleep even at lower volumes. I actually gave up on adjusting the sensitivity and just bought a signal gain inline amp and fed it back to an unused channel on the amp.