X can v2 outputs to test valve response

[musicmanic]

I would like to test the frequency response of different tubes through the rca and headphone outs of the X can v2. Seeing that the headphone outs are looking for high impedance headphones could I feed the rca input from there? Would that be safe and will there be no high frequency roll off?
Also are the rca outs of the headphone amp direct outs or do they go through the circuitry of the board and tubes first?

Thanks