Agree with Csontos
(1) If you capriciously hardwire in a new power cable, you may trigger a safety issue. If it causes a fire or damage, you probably violated / nullified your insurance coverage
(2) Without prejudice to the above , IMO the potential performance uptick of doing it in a receiver without the higher end power supply, likely just isn't there.
(3) Add to this the added cost of having an IEC female plug dremmeled and soldered in professionally ( if it can even be physically done with the crowded layout in a dated receiver) and then purchasing a quality external power chord , again likely isn't worth it.
If it's not working for you, trade up to new gear instead .