I do not know if the RS adapters contain resistors. They have worked well enough for me, though, the few times I've used them.
The noise might suggest the RS adapter is a simple Y, without resistors. Otherwise, I haven't a good guess to explain the noise.
If you consider the 1k resistor to be in series with the power amp input, which is likely tens of kilohms, then you see any level drop is fairly insignificant. And the resistors do isolate the channels from each other. "Isolate" in this instance may just mean that one channel doesn't try to drive the other channel.
The resistors will cause a change in impedance. Normally not enough to fuss about. But it is one of many reasons why using an op amp adder/mixer is often the preferred method of combining channels.