Yeah I have one of those great little GB heads. When I'm using backline-du-jour I always bring a speakon-to-quarter-inch cable I made, in case there's a house cab that doesn't have a speakon input.
I've seen other modern heads that lack quarter inch outputs too.
I always wondered why XLR connectors didn't catch on for speaker cables. Trace Elliot used to use them.
They lock, they're everywhere, they're sturdy, and they can take a fair amount of power.
Apparently, based on some googling, there was never a standard for which way to wire them up; and they can take up to 14 gauge wire and 15 amps, which while it's quite a lot for most purposes, the current rating might be a bit low for some PA rigs. Speakons are rated to 25 or 40 amps depending on the specific plug, and some can take 12 or even 11 gauge wire.
Of course, for bass players plugging a one or two meter cable between a head and a cab, that's all likely rather excessive. I was curious what it'd take to get near 15 amps through a single cable. 900 watts (running flat out) into a single 4 ohm cab would do it. I would prefer not to be very near a maxed out 900 watt amp anyway :-o