New Power Cable on a 1990 Hafler DH120?

The sound of an electronic component starts with its power supply. NOT with the wire connecting it to the AC outlet. If you believe otherwise then I recommend joining the Flat Earth Society.

Power cables do make a difference,  If you haven't heard it you either aren't listening or didn't try the right one.  You will have to solder the connection to the amp board and a large diameter power cable will require you to probably drill out the case and find the right grommet.  The GR is good because you can specify only one end but getting the other to match up is going to take some work.  

For my Dynaco amp I used an inexpensive heavier gauge power cord because getting really large wires or cutting the case to put a plug adapter was more than I was prepared to do.

Does it have an IEC connector? If so, it is Easy to start with a generic one off ebay for less than $50 and see if you hear a difference. read the fine print to make sure it is 10ga. I like the woven GR research cable.   I think just about all amps benefit from a larger power cable unless the power supply is choked by a long 14 ga wire in the wall or a power conditioner.

If you want to experiment buy an IEC outlet. Cut off most of the old power cord, leaving several inches. Strip the two conductor ends. Attach each copper conductor to a screw terminal on the back of the IEC outlet. Now you can use any power cord with a male IEC connector. 

@carlsbad : those old Hafler SS amps/preamps were made before IEC outlets were in common use. I have a Hafler 220/110 pair - both with captive AC cords.