AGC derived squelch on HF is not very successful, especially on utility stations where it would actually be most beneficial. This is because the squelch is usually driven from the same circuit that drives the "S" meter which is derived from the AGC - the measure of the strength of the signal. If there is plenty of signal, then fine, the squelch will often work quite well as you can back it off sufficiently to kill the noise. SSB stations don't have a carrier to drive the "S" meter so there's nothing to lift the squelch until someone speaks and often the first syllable is missed.
But there's the problem - in a VHF radio most of the noise comes from within the radio itself plus just a bit of cosmic noise so it is relatively constant and the voice signal many more times larger than the noise. But on HF most of the noise is atmospheric, static crashes and bangs, interfering signals etc often much higher in level than the required signal so the squelch will open on the signal you don't want and ignore the ones that you do!
The best solution is a radio with an RF gain control which you can back off so as to reduce the noise but still hear the voice.
Second choice would be a (expensive!) radio that has audio-derived squelch - it listens for speech frequencies and patterns, analyses them in a bit of software and then sends them to the speaker if they comply with the algorithm - often with a bit of a delay where the signal is stored while the analysis is going on.
...and of course you mustn't ignore the MK1 earhole - it's very good at ignoring background noise and picking up a voice or morse code buried underneath! My old Kenwood R2000 has a squelch control and I fitted a mod to give it an RF gain control - guess which one I use for the HF aero that I listen to most?