Sonos Amp to 3rd party sub: subs turns off randomly

I’m not sure I buy in to the statement that an RCA splitter shouldn’t cause an issue. You’re reducing by 50% the signal received by each Sub’s line in. Have you tried a powered splitter, rather than a ‘simple’ wired one?

There would be slight losses, IMHO, caused by the length difference of the cables, but I wouldn’t ‘think’ that to be significant, certainly not as much as the splitter. You could test this, though, by getting equal length cables, and replacing your current ones. Might be a good test anyway, in case there’s an issue with one (or both) of the cables, causing the interruption. 

Appreciate your response. 

• I am going to try with same length wires and see if that fixes it.
• I wasn’t even aware of powered splitters. Do you have any recommendation of a product I should get?

Thank you!

Normally a Y splitter is no issue as the source is low impedance and the destinations high impedance. There is a minor, usually unnoticeable volume drop but I have never seen an issue. But I don't think I've seen one on more than a 3 foot cable.

The long cable may be a problem as it could pickup noise and feed it back as well as forward.

A powered splitter would isolate the long cable from the Amp as well as the other sub. If you can't find one look for a powered preamplifier that could be used as a splitter.

I’d be interested in what you find out by replacing the cables. And I don’t have any significant experience to gainsay ​@Stanley_4 ‘s comment, either. I just don’t use any splitters, haven’t for the last 30 years or so, if I were doing research, I’d start on Amazon, or look at a google search. 

Sure, I will update the thread with whatever I find out. Thank you both for your valuable comments.

25 foot cables and splitters would impact a phono signal, but have no significant effect on subwoofer signals.

Are the subwoofers shutting down? As a test force them to be On, rather than Auto.

If you are powering the subwoofers through a single surge suppressor, it may be shutting down when both are operating.

A quibble on impact on the signal. 

A phono output (cartridge level, not preamp output) has a very limited (see the cartridge spec sheet) ability to drive the attached cable and any attached devices, both in voltage (fighting resistance) and current (fighting capacitance) making both cable and device specifications critical. In addition to this the needed phono RIAA equalization and signal amplification make the cables noise (50/60 cycle hum in particular) rejection even more important. 

In comparison the ljne-level signal is much higher powered, Sonos doesn't release much information on the audio port, in or out, specifications, so getting an exact picture is difficult, about the best you can do is use generic line level port specifications and hope they are close. Given that, the main issue seen with longer cables is hum as the output ports impenance is easily able to drive the cable and one or two attached inputs. Probably more than two but without hard specification numbers from Sonos a cautious limit is good.

I am also interested in hearing what the issue turns out to be. I'm hoping it isn't caused by the Sonos output being less capable of driving the load than expected.