There are no analogue or digital Outputs on a Connect:amp, it’s never going to act as a streamer to an Integrated Amp; you require a Connect or Port if you wish to stick with SONOS.

There are no digital, or analog-out ports on the Sonos Connect-Amp.. so not sure how you plan to connect it to your 3rd-party Amplifier? See attached image.

What will the integrated amp give you that the Connect Amp does not?

@Kumar a Connect:Amp vs. Connect (or Port)+Integrated Amp gives you far greater choice of loudspeakers to suit your Room, there are lots of Loudspeakers you would not partner with a Connect:Amp.

There are just as many that work fine with a Connect Amp that is as good as any HiFi amp of 55 wpc - I use a Quad pair and Dali pair just now with two, and in the past I have even used Harbeth - and never needed to have the volume level slider on the Connect Amp past 90% while even 100% is distortion free. On the other hand if you have or simply must have speakers that suffer impedance drops to sub 4 ohms levels and/or have low sensitivity, then the 55 wpc output of Connect Amp may not suffice. For such I would go past even the Sonos Amp and look for a beefy power amp of 200 wpc + and wire the Port directly to it using the volume controls on the Port.

Need to connect devices like a turntable or a Sub may call for the power amp to also have a pre amp upstream of it.

And a Connect/Port + any number of Integrated Amps will give you more options and improved performance with your Quad, Dali or Harbeth Speakers. 
 

As you say a Connect:amp ‘will work’, but it can also be a limitation on the overall fidelity of the system.

There is nothing objectively established anywhere in the world that makes the Connect Amp to be less capable than any other amp of the same power delivery, in a listening test or via published specs. 

I have used amps from ‘HiFi’ makes in a past audiophile life with the same speakers and found nothing missing on replacing these with the Connect Amp.

I no longer believe in claims of golden ears. 

Our experiences differ!
 

Back to the OP, the Connect:amp is not the correct device to use with an Integrated Amp.

Indeed. And the OP still has not said why he needs to replace the Connect Amp with a lesser amp - lesser in being one that does not have the streaming capability that the Connect Amp does.