Any good for use on low bass ?

I did look at that, unfortunately the 4 ohm versions have very low Qe, meaning there's quite a sag in the bass range, so even though it'd draw more power being 4  ohms, it doesn't keep up with the 8 ohm 15".

I also looked at using 2 of the 8 ohm version; while that gets louder than the 15" it doesn't get as deep, unless the cab can be made bigger still.

Two of the 10CL51 would be hilariously light and get about as loud & low as the 15", though would be a bit more ££. 

Not as suitable for upgrading the amp later however as they'd already be close to max excursion on the existing amp.

I actually do think that the 15CL76 in the biggest box that Slaz can afford to make out of the Hexalite panels (and indeed carry up & down the stairs), is the sweet spot right now, FWIW.