As you know, most integrated amplifiers have small U-shaped jumpers
between the pre-amp section output and the power amp. section input.
I presume this is to allow optional signal processing to be added.

I had reason to do this under two separate scenarios, as follows:
1. On a Yamaha RXV1070 receiver to add a high quality preamp to for my
CD player (didn't want the RXV1070's built in audio effects on CD,
just on home theatre.)
2. On a vintage Sherwood S-402 CP amplifier to add a subwoofer
amplifier driven by the pre-amp output. This unit, a PartsExpress
model 300-804 (recommended), has an I/O group that passes on the
signal unchanged to the power amp. section, while picking it up for
the sub. amp. .

After doing this, in both cases, I noticed a very low 60 Hz hum where
it did not exist before (I was using high quality audio cables.)
After a bit of forensics I concluded that I had introduced a local
ground loop around the extra cable shields.

To fix it, I made up one cable pair (male phono to male phono) with
the shield NOT connected though for use in one, only, of the links (I
chose the pre-amp output link to the "300-804".) The result was no
hum at all. Indeed, you could prove the method by re-making the
missing ground link with a piece of wire and hearing the hum come and
go.

I labeled the special cable pair to avoid future surprises!

Cheers,

Roger