Hiya Howie! All is well here. Hoping the same for you.
Re: MarkBass 15" cabs, I really haven't spent any time with them as I'm not a fan of the form factor of most 15s due to the size/bulk. Kind of clumsy for this old man. I'm more familiar with their 10s and 12s and I'm hesitant to recommend anything that might be considered "underwhelming" compared to your Mesa 2x15" cab which I'm sure is a killer unit.
That said, the Traveler 151p has gotten a lot of love historically. It's fairly compact for a 15" and handles 400 watts @8 ohms so it would work with your Genzler head.
It's the most traditional of the MarkBass cabs as it's still constructed of wood.
If I were buying "blind" I'd give that a chance. I'm sorry I can't be of more help on this one, Howie. I'd Google the cabinet and read people's experiences.
Best of luck with your choice and let me know what you choose!
Steve