Hi Walter,
I beg-to-differ with you on this issue. I under stand where your are coming from so let me explain.
A while back I had a 1959 T100D with Franz pups. It was a beauty with a 9/10 finish, good pups and great neck.
The thing I didn't like about it was it howled like crazy. I use a lot of hand dampening so am familiar with the technique for feedback suppression. IMO the T100's from the 1950 and 1960 are one of the most howling feed back guitars Guild made. The mid 1960 Studio guitars are also in this class. IMO the SFIII is less prone than the T's and Studios. I suspect it was the the thin top and possibly less sound board bracing.
I speculated that the ideal situation would be a semihollow body guitar with Franz pups. I checked "The Bible" and inquired to Hans for any Guild semihollow, semi solid or solid body guitar with Franz pups. We came up blank. The Franz pups were phased out about the same time Guild was getting into semihollows, etc.
So I decide to make my own "Guild" semihollow with Franz pups with a DeArmond Starfire Special. I posted a picture of this guitar a little while ago.
I can tell you it was an excellent configuration! The neck position is very full and rich sounding with absolutely no feed back howl. I experimented this morning sitting close and in front of an amp (Thunderstar) and cranked it up with no problem and the tone was excellent. I needed to turn away when I switched to the bridge pup or it would squeal like any other guitar. The high end was bright and sharp and could really cut through. This was too bright and sharp for most applications (i.e., the Stones "Sympathy for the Devil" type of riffs) so I lowered the treble of the amp to manage it a bit.
So I think you would be suprised with the performance of the Franz pups in a non hollow body guitar.
In regards to your particular experiment with a DeA X135..well IMO the most critical sound/tone test for any model guitar is always a hollowbody. I owned DeA X155's at the same time I owned a Guild maple top X375 and there was no mistaking the Guild for the DeA. Both were full 17" lower bout guitars and the tone differences were like night and day. The Guild had a tight deep low AND bright tone. The DeA was flat on the low end and tinney on the upper end. When amplified you could still easily notice the tone differences but you could compensate for some of the difficiencies. (Like a fool I sold the X375 because of bridge feedback issues before I knew it could be fixed with wax potting.)
Now I have noticed the hollow body comparison test is easy to discriminate the tone of a higher quality guitar. However as I compare Guild verses DeA semi hollow or solid body guitars it isn't so easily clear which sounds better. I did a casual comparison of a 2000 DeA SFCustom to a 2000 SF5 and it isn't so obvious. I tend to give the Guild the edge but I know the results and can't eliminate the psychology bias in the experiment. The guitars are not setup the same and the strings are aged differently, etc. so to do an accurate test would take bit of an effort.
So where I am going with this is that your situation was more critical or sensitive to tone differences than for non hollowbody guitars.
M