My problem with plastic cabs is that, while they sound OK with full-range music, when you only have bass going throught them, they sound like you're playing in a bucket. I'd expect the effect to be much less evident with a smaller cab like this one.
I strongly suspect that the dip in the FR at 1k5 is a cabinet artefact, which will disappear off axis. So, I wouldn't worry too much about correcting it.