I used to run a Walkabout with a fairly efficient, mid-loaded (and mid-forward) 2x12 cab -- 100 dB/W/m just for the woofers -- maybe a little more, overall, with that forward mid. Plenty for many situations (I can see a Walkabout into even a moderately efficient, 8 Ohm 1x12 comfortably covering several smaller settings) but, playing loud music with loud people in a loud club, I was deep into full-on overdrive with that combo. An fDeck HPF3 in the effects loop helped substantially, but it was not enough.
If I were to try to cover the same sort of thing with a Walkabout today, I would want two fairly efficient 15s or something comparable. Going bigger, two good 2x12s (or, again, something equivalent) would make a massive difference vs. one, and would sound huge. An HPF gives you some more room, yet, and can be used with whatever cabs you like.
Granted, EQ choices will make a difference and some 15s aren't any more efficient than 12s -- I'm speaking generally, with the point being that, if you're trying to play loud and stay clean-ish and dynamic, the WA really benefits from an extra few dB of efficiency. In terms of cab voicing, I found good mids (cab doesn't have to sound ganky, but a big scoop or hole in the mids doesn't help) and a tight-ish low-end response to be the Walkabout's friends, but I suppose that depends on what you're going for.
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Addendum: I'm never sure when to use the term efficiency vs. sensitivity. I had the notion that, in speaker-jargon, they might not mean exactly the same thing. A quick search turned up this, which I found informative (and even mildly entertaining): Speaker Efficiency vs Sensitivity | Types of Speakers
Addendum 2: Though not what I usually go for, the WA's overdrive when pushed near its volume limits (not just dialing up the gain) is some of my favorite that I've encountered -- harmonically rich, smooth, creamy (I feel like I'm talking about something you'd get for $8 in a fancy coffeshop) is how I recall it.
