Unfortunately Richie, your location makes it tough, but yes, I too would recommend trying to source a single 075.
Post a Wanted Ad here and see what happens. I'm sure someone here would have one. Worst case scenario, is that I know someone who might be persuaded to sell you one, but it's in Australia, so postage would be really expensive.
I'll ask him in the meantime to see if there's one spare and the cost, but it'll be working at least and highly likely to have an original diaphragm. I'll know more tomorrow as it's night time here now, and everybody [except me] is in bed…..
As far as what you have, it would have to be an AlNiCo, only because those boxes are older than when ferrites came out. [I think…. anyone feel free to correct me if I'm wrong.]
Now as far as the 15" are concerned, try to locate a pair of JBL 2215. They come and go. They're actually one of only a couple of woofers from that series that have an fs of 20Hz.
Depending on just how loud you want to take these things, I feel they'll be more than adequate, and the 4320 is actually tuned to them to start with.
Forget about the Lenses for now. EVERYBODY I know, including myself has soon learnt that the box sounds great without them.
If you're mainly a single person listener, then dispersion is the last thing you should be worried about, as you just end up sitting in the 'sweet spot' anyway, so no need to worry about what it sounds like 'off axis'.
I'll get back to you in a day or so about the 075….
Cheers.
ps: I will add, that it's the box that makes the bass sound what it is. If you're a low level SPL listener, you could just about throw anything in the way of a 15" woofer in them, and they'd still sound great to a non-critical listener, and you just might be happy with it until you could find and afford the real things. At least you could still be enjoying the use of them while you're waiting….
[let the arguments begin…. LoL]