Here's a response I received from Peter Murry, the Lusturous Potentate of all things Markbass in the US:
"Hi there,
An extension cab will definitely give you more headroom and more bottom end, and will handle your low B better at higher volumes.
Since there's no crossover in the combo's head, it's impossible to bi-amp... so any extension cab you choose will be full-range.
Inevitably the bigger the cab, the bigger the sound and the fuller the lows. It's a myth that bigger speakers give more low end, it's more to do with the total surface area of the speakers and the size of the cabinet. So more 10s might actually give you more lows than a single 15.
If you want to avoid adding more highs, one good thing is that any extension cab you choose will have a tweeter attenuator on it so you can just turn the tweeter off.
If you don't already use it, the VLE is also your freind if you want to trim highs from your sound.
Any extension cab will give you more lows... and the line array phenomenon increases lows too, which is an argument in favour of more 10s...
A Traveler 102P would be the most portable option, but the bigger you get, the more lows and headroom you'll have... so the Standard 102HF would be then next, then Standard 104HR, then Standard 104HF.
A 1x15 would of course be another option, but it wouldn't give more low lows--more of a low-mid emphasis. And wouldn't add as much headroom as the last three cabs mentioned above.
I hope that helps, if you have any follow-up questions, please let me know!
Best,
Peter"
I really wanted a 151P, but after reading what Peter says, I think I'm going to go with a 102HF