Expanding 9.2 to 11.2

[RJ MacReady]

Hi all, I just got a Pioneer VSX-LX503 9.2 channel receiver, upgrading from my Denon 7.2.

I understand that you can expand to 11.2 if you run a line out to a separate 2-channel amp.

But my question is, can you use an old 5.1 receiver instead, or does it specifically have to be a 2-channel amp?

I have an old Sony 5.1 workhorse (so old it doesn't have HDMI inputs). But I'm guessing I can just run the line in to one of the RCA inputs on it, connect the two rear Atmos speakers to the main L-R inputs, and that'll work?

If not, anybody have recommendations for a 2-channel amp that wont break the bank?