OK bit of a belated update.
The 5210C alone was OK but I plugged in the Ampeg for a bit. Not happy: to my ears at least all got a bit heavy & muffled.
So next thought was a Hydive 4x10 as extension. Found a post saying how that would be unbalanced because the 350w into an 8 ohm 2x10 would turn into a 4 Ohm load, being 250w into a 2x10 & 250w into a 4x10.
But that sees wrong.
If I have got this right, end up with a very odd load because lashing the 4x10 on gives you
2 x 16 Ohm 10" units in parallel (the 5210) which then has connected in parallel
2 x 8 Ohm parallel wired 10" units connected in series to another set of 2 x 8 Ohm parallel wired 10" units (the 410)
So in effect 3 sets of 2 x 10 inch units wired in parallel to a common amplifier feed, but 2 are 16 Ohms an 4 are 8 Ohms and the net is 500w into 4 Ohms......
Before I go lay down in a dark room for a while:
a) Is that right?
does it matter?