Some Ten-Tec rigs required a particularly low resistance to
ground to key the rig, due to the analog circuitry. A pair
of earbuds probably wouldn’t work. I used to have to
clean the contacts on my straight key regularly.
That’s not to say that the IC-551 has the same issues,
but rather to observe that different radios have different
requirements, and a resistance that will key one rig might
not key another reliably.
On the other hand, the old Argonaut used the same line
for the key and the PTT lead, so I could use the microphone
button to send CW (as we used to do with AM rigs to make
CW contacts on Field Day).