in my big rig i use 4x cubo15 and they are Kappa Pro LF loaded and i like the way they sound for kick and sub... the tham15 on the other hand is a slightly different beast. not better or worse, just different. i cant suggest a driver to you as i dont use them (at the moment) but if you nip along to the site of the designer he has a few suggestions and even a few graphs to show you what he has tested and what other people have tested. but it is a folded horn so you'd be looking for a driver that works well in horns as appose to reflex.
anyway..... a little reserch on the tham15 and drivers should steer you in the right direction.
saying all this, depending on budget.. i get my amps from MATRIX AMPLIFICATION and a few drivers from TONY WILKES on these very forums. so, you might want to ask Tony about a good single driver for a single tham15. i might even suggest a 4ohm version as appose to an 8ohm one so you can use the amp to its fullest extent and then you could pick a slightly cheaper amp as an amp that does 800/800 @ 8ohms will be more expensive (if buying from the same brand) then a 800/800 @ 4ohms.
i.e.... if your two top speakers are 300w each @8ohms hooking them up to a single channel on your amp would mean the amp would need to be able to drive a 600w load @4ohms (its just how the math works with these things) so using this principle means you would aim for an amp that did about 600 per channel at 4ohms but because it 600watts @4ohms the power it would use to drive a single speaker on the other channel be it an 8ohm sub... the power rating normally is dropped to, say 400w as the load is 8ohms not 4 like the other channel.
soooooooooo..... if you bought a sub driver that was a 4ohm driver instead of a 8ohm driver you would then be using that channel on the amp to its fullest extent.
...let me know if all that make bugger all sense and i waffled way to much.