If it's the cab that originally came with the amp, it was loaded with Sunn Transducer labeled woofers, probably CTS bass drivers. What you load it with going forward should depend on what you want it to do, probably nobody can give you real accurate advice on getting that "tone in your head" that most players are searching for; after all, it's in our head. If you can find sound bite examples of what you want, and share these, people can probably give you some direction.
Your Sorado is a strong 60 watt, clean, moderately high headroom amp designed for bass guitar use. They will get nasty, but the stay clean pretty far up the SPL level compared to say a Fender or Marshall rig of comparable output. As you have discovered, even though it's a pretty "hi-fi" amp, it takes pedals well and as such is very versatile for guitar.
The fact that you are looking for 15" guitar drivers narrows your search a bit, as with current production guitar speakers 15's are lees common than 12's or 10's are.
I'd say that pound for pound it's hard to beat Eminence for tone and value, also WGS Speakers are a quality, reasonably priced driver.
WGS also make an AlNiCo 15" that is kinda expensive, and I've never heard it in person, but...
I have a lot of only positive experience with WGS, and the word is that it's a killer 15" guitar driver; they can't build them quick enough to stay on top of demand. I personally have enough respect for their company and products to drop $500+ on a pair if I were to decide to convert a Sunn 2X15 bass cab to guitar use, however at that price point I'd stop short of trying to convince someone else to, without personally having "hands on" experience with them.
Of course, a pair of vintage JBL D130F's wouldn't suck either!
