Not entirely sure stock Sunn Transducer speakers were CTS or Eminence (Ed and Issac seem to be conflicting, and I've been under the impression they were Eminence not CTS to keep costs down, like Ampeg). However, Issac is totally correct about the Sunn Magna and the Cerwin-Vega being the proper upgrades for your cabinet. That being said, I don't know anything about the Magna or the C-V. As for EV speakers, I've only seen them in later, Hartzell era cabinets. EVM15B Series II (8 ohms, 200 watts) specifically. I had a pair in my 215B, good speakers and supposedly not any different than the originals. Also, correct me if I'm wrong; but I don't think a speaker from 1978 would be in a early-mid 70s cabinet stock, by 1978 the 115S is most likely out of production.
I don't think it's a necessarily a conflict really.
Many industries cluster in a given area. For example; there is (or maybe more accurately "was") an exceptionally large number of major RV manufacturers in Elkhart Indiana, the same with tool and die manufacturers in Cincinnati, Ohio, furniture and fabric manufacturers in the Carolinas, automotive in Detroit, etc.
Because of this, industry specific parts source suppliers, industry specific experienced workforce, and the other attendant resources also cluster in those areas, making logistics much easier for
all of them.
In Kentucky, particularly the far west corner of Kentucky, there is/was such a "cluster" of speaker manufacturers and the attendant resources. Both Eminence and CTS were headquartered there, and in the 50's, 60's and even into the 80's (until inexpensive Asian imports took over that market) most of the throwaway speakers in radios, tape recorders, and "you name it" came from there.
They also made private labeled
higher grade speakers for the majority of US MI manufacturers like Acoustic, Ampeg, Leslie, and... Sunn.
CTS was first, Eminence was an off shoot started by ex CTS employees. They stayed friendly, there was enough business for everybody, it wasn't a "cut throat" situation, and who actually made what from about 1966 to 1986 or so can get REAL hazy; they all bought magnets, baskets, etc. from the same suppliers, and sometimes even one would build speakers for the
other's contracts. Even the manufacturers code stamped on the speakers gets convoluted.
This is a fairly common practice in the other industries I mentioned above also.
So, the early Sunn transducer labeled driver was basically also (just to name a
few) the Leslie 122/147 woofer, the Ampeg B15 woofer, and very likely the Acoustic driver in bigo's cabinet, and could actually be an Eminence
or CTS.
Now, here's the next source of confusion on this specific subject. At some point Hartzell started building their
own speakers, and one of their facilities was in
Kentucky to take advantage of that "cluster" thing I spoke about, buying parts from the same sources as the other guys and cooperating in the "scratch back" manufacturing atmosphere that often exists in these "clusters".
So what is a "Sunn Transducer"? Got me, depends on too many variable to pigeon hole it!
I suppose it doesn't really matter, aside from some cosmetic difference the specs were consistent from builder to builder so the performance between them should fit in a narrow window.
As a side note, manufacturers of high end
premium drivers; JBL, EV, Cerwin Vega, used proprietary components and specific construction techniques, unlike the all for one, one for all practices of the more pedestrian builders.
This concept is also true of "premium" products of many other industries.
