Had, in TWOK, Kirk ordered to raise the shields as Saavik/regulations suggested, the Reliant wouldn't have been a problem for the Enterprise. Because Kirk + Spock + fully functional ship >>>> Khan + fully functional ship. They would have come up with the prefix code idea as well, but with full power.
So Khan's surprise attack could also count as pretty embarrassing.
It's kind of a wasted opportunity that in The Search for Spock Kirk did not have to answer for that.