For audio, careful about the HexFETs, or any low voltage VGS logic level mosfets. They are designed to switch, not operate in the linear region as the sharp low voltage turn-on, beginning at around 2.5v, doesn't evenly turn on all of the silicon mosfet die structure at the same time, hence the Safe Operating Area / SOA is actually worse than older mosfet with a higher voltage VGS as the larger slowly switch on voltage of the mosfet's cells mostly begin to conduct more evenly throughout the entire silicon die.
Doesn't matter, the SOA is what it says it is.
I've tested IRF740PbF for example, at voltage, and found it free from 2nd breakdown at DC. Most datasheets unfortunately do not show DC SOA (though older datasheets may have had them). I wouldn't suggest using them if you need design guarantees, but I would recommend testing them for DC SOA at your required operating conditions if you are willing to do a qualification test / process.
Shockingly, modern (SuperJunction) types often have full (DC) SOA, despite higher power density than ever; I don't know what it is that actually makes it work, but I'm not complaining.
Note that many also do have 2nd breakdown, sometimes quite severe (I ~ 1/V^2 or worse). There's no generalization, just look at the datasheet and see what it says. If it doesn't have the SOA you need (time or power), keep shopping. Simple as that.
I've also seen some people harp on about "Spirito effect", with a partially-formed argument based on gm tempco -- but transistors don't automatically fail from positive tempco. There's more to it than that. And the only way to show that, is to plot the SOA proper.
Tim
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