Jay,
Based on my experience, I'd say there are two choices. Something
from PS Engineering or whatever is in second place. Yes, the King KMA
28 is provided to Honeywell (...Allied Signal...Bendix...King, yada
yada yada) as an OEM product designed and manufactured by PS
Engineering. Likewise the SL-10 and SL-15 from Garmin-AT (...UPS
AT...II Morrow...Apollo, yada yada yada) are also designed and
manufactured by PS Engineering. They may great audio panels and
intercoms. Their reputation for quality and support is a model of how
gen av folks should run their business.
I am flying behind the same stack that came with my plane when I
bought it four years ago. The whole stack is vintage 1980 King Siler
Crown, including a KMA-24. No complaints, and not a single need for
repair yet. The intercom is a six place PS Engineering. I bought my
previous plane without and intercom and installed a PS Engineering.
Eventually, I will be upgrading the KMA-24 to a PMA 7000B. I've
worked in aviation for the past 24 years, and have personally
integrated their products with air-to-ground cellular and satellite
systems.
Since you already have a PS-Engineering intercom, the most costly
part of your wiring has already been done, the wires from the panel to
the cabin audio jacks. The easy way to wire it is to have an
interface cable made up that can be put in place. That is, get a
mating connector that is the same as the one on the back of your
intercom. The PMA 7000B has two slide in connectors in the tray. The
top connector is the intercom, which is the what you want for this
harness.
The audio panel wiring that you want to remove from the existing
unit all wires to the lower connector. Slide the Narco intercom out,
remove the tray fasteners and pull the tray out as far as possible.
Then find out the pinout and get a one to one correspondence between
the power connections and other avionics. The key is that all of
these wires are already connected to your radios, and conveniently
located at the back of the audio panel tray. Here is what you will
likely need to re-connect:

1) Connection to the marker beacon receiver: 5 wires and one coax
2) Two VHF comm radios: 5 wires each, TX pair, RX pair, Gnd
3) Audio connections from two VORs: 2 wires each (might be the same
GND if you have a Nav/Comm)
4) Audio connection to the ADF: Two wires
5) Audio connection to DME if applicable: Two wires
6) Direct wired pilot mic/PTT and headphone: 6 wires
7) All of your other wiring to mic and headphones, which go from the
existing intercom to the top connector on the tray, and can be made on
the bench.

All in all, one harness that can be made on the bench for the
audio, and about 30 more wires and one coax that have to be done in
the plane - but they are already there. Remember to use twisted
shielded pairs on all audio lines, and only bond the shield to
airframe at one end, preferably the end that the audio comes from.
Even that may already be taken care of pretty well. All of the nav
radios are only RX at the audio panel, so should be bonded at the
radio end. All you need to deal with is bonding of the two VHF comm
TX pairs.
It may not be trivial to get to. Mine is not too bad because the
audio panel is at the top of the stack and I can remove a cooling
grate on the top of the glareshield. It's worth talking this over
with your radio shop to see if they agree. They could probably do the
pre-wiring on the bench in about an hour. Re-connecting the radio
wires to the new tray connector will depend on accessibility.

Good luck,
Brian Cox