Headphone Amp. Where to hook it up??

If you want to use the volume control of your integrated, than you'll have to connect your headphone amp to the integrated's pre-out, if available.
If not, then you may connect it to the tape loop out and use the headphone amp's volume control.

Use the tape output or other fixed-volume output. Headphone amps often have a pass-through so you can still use the tape out for taping.