The IS-1000 is having the AMP IN input selectable on the RCA Input 2 that converts the unit into a power amplifier (Fixed pass-through Output). IS-1000 also has Fixed Pre out and variable which you can connect to any device at the same time of the speaker output.

Yes the subsonic filter in the PH-10 is always active and as you know at such frequency is totally not audible. The 20Hz roll off is part of the RIAA design so necessary part of the structure.