Not trying to get in the middle of that, lol, but I have to say that I was really skeptical of my cab years ago for guitar (2x15 Celestions) with a '69 Silverface Bandmaster Reverb head but it sounded really good. Still though, for a pedal steel, why go 15"? Not trying to blaspheme as I know how good those JBL's are but you're not afraid of bottoming out with a 15? Instead of a 1 or 2x12?
I'm not advocating either way, intellectually curious as I don't think I've ever heard a pedal steel through 15 inchers. That I know of, anyway.