So all I had heard from the cabs was a 30Hz test tone but I don't have an amp yet to make them do what they are supposed to do. Time to modify the 15W practice amp! Disconnected the speaker, extended the cable and added a 6.5mm plug - Cue bad photo of a mini rig of Doom:

it isn't but it's better than nothing and I can play my bass through them.
I've since had a friend bring his Peavey Firebass 700 (700W into 2Ohms, 475W into 4Ohms) and that was a better test with both cabs. No way would I ever want that head, it weighs a tonne (23kg/51lb really). Both cabs wouldn't weigh that (don't quote me on that yet, still to weigh them). I wasn't fussed on the tone either.
Without wanting to fork this thread to a amp discussion, here's what I'm thinking amp wise (all IMO and I am not very experienced in the ways of the tone, just like what I like - metal like Iron Maiden, rock like Guns n Roses, AC/DC and a lot of other stuff that doesn't fit with that!)
I've been looking at a few micro heads such as the Ampeg PF350 and PF500, Mark Bass TA503, Genz Benz Shuttle 6.0 and Streamliner 600. I have only tried the PF350 and Shuttle 6.0 thru a single Jack 112 (all others were before I had them working and so were with cabs in the stores). I liked the PF350 and having tried the PF500 previously I think that would be better, more featured and versitle, higher output for two cabs. I really liked the GB Shuttle 6.0, just sounded great, easy tone and easy to change from clean to a bit of bite. I was disappointed with the Streamliner 600 when I tried it through the Mesa 4x10 cab in the store (same cab as for the PF500) but I want to give it another go with the Jack. I didn't think the lows were articulate enough for me. I have asked the store to see if they can get a GB Shuttle 6.2 as it is supposed to sound even better than the 6.0, more tube stage gain available. Am waiting anxiously. Again all IMO, YMMV AFAIK and that ain't very far.
Built: 2x Jack 112.
Bass: Cort Action A.
Amp: GenzBenz Streamliner 600.