YES, there will a capactance load to a linestage O/P impedance, active or passive regardless. Passive linestage can most likely get more HF roll off due to the IC capacitance loading on its relatively high O/P impedance & low signal current flow.
With or without any headphone loading regardless, always take away the stand-by IC btwn the linestage & the head amp & vise versa.
For passive linestages, NO, repeat, no parallel running ICs, shorter the IC the better to reduced the capacitance load to miniumum to prevemt HF roll-off.
If the I/P stage of your power amp or your headamp employ pentode
instead of triode, then the capacitance issue of the ICs will get much less effect as the I/P capacitance of the pentode is generally much lower than triodes, which are commonly used as I/P stage of amps. Check up your amps first.
As in my situation, I got a stock Dynaco ST-70 which got a pentode
I/P stage & got no problem at all driven by my passive linestage
via a 6-ft long 4N silver ICs. NO way I will get any ICs running parallel to other power amps.
c-J