Effects loop return is just another term for power amp in, so it's technically the same thing.
You shoud avoid running a line level signal out of the slave into another amps input. The slave knob adjusts the strength of the signal. So it's essentially there to match that output with another input, so level-wise, when keeping it really low, you should be able to match it with another amps guitar input (which is designed to suit a guitar signal, which is much weaker than a line level signal). If you have the slave out control too high in this scenario, you'll experience clipping (like with a boost pedal, which essentially does the same thing).
I'd try and use the seconds amp master volume to control the overall volume, and use the slave out control to simply match the levels. And if possible, try to use the power amp in/effects loop return, as the signal will be least colored this way. Any additional preamp tube ciruit will color the tone, set clean or not. And most tube preamps have four or five, so go figure.
The slave is designed to work with line level inputs as well, so a power amp in/effects loop return is the way to go.