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.
With the exception of TMP and maybe TVH, the TOS movies seem predicated on the TOS crew and Starfleet being bumbling idiots. TWOK was the worse of the lot for this.
Fine Enterprise is the only ship in the sector--why the Enterprise is hanging around on a training cruise in a sector that has both a top secret research lab and Kirk's old girlfriend, who knows, but it is. Starfleet should have figured out something was up and sent out an extra ship or two to back up the Enterprise. "Could be nothing, garbled communications"...from your top secret research lab that's building a super torpedo that reformats planets. I think that's worth sending a cruiser or two to back up the outdated training ship full of cadets.
Kirk defies all common sense, even after a junior officer tells him they should raise shields; to top it off Spock calls her down on it. The Enterprise crippled, a lot of the TRAINEE crew wounded, power failing, Kirk decides to continue on to Regula instead--oh I don't know--hauling ass to a starbase or getting out of jamming range and getting a squadron chasing down Khan.