Your D140 is probably at least
40 yrs old, it may be that it's just tired. The suspensions on a driver wear down and loosen. Reconing is a fact of life in the speaker world, just routine maintenance. Until recently I worked for a touring production company that ran stadium sized L-acoustic K series rigs, when they came off the road there were stacked pallets of drivers that needed to be reconed. Some had blown in use, but the majority of them were worn by hours in service and were getting "soft" so were reconed as preventative maintenance.
Modern drivers will take a bunch of power, but generally sacrifice tone and sensitivity. In the SR world accuracy and SPL are the keys to success, and nowadays watts are dirt cheap; so heavy surrounds, spiders, double voice coils/spiders, etc., are commonplace. You can beat the snot out of them and they will take it, but they need
huge power to get to to acceptable SPL's.
In the era of the D140, a 100 watt amp was considered a MONSTER. Drivers needed to get all the SPL they could from the low powered amps of the day; diametrically opposed to "You can beat the snot out of them and they will take it, but they need
huge power to get to to acceptable SPL's".
But we are talking about something else also. The D, K and E series JBL's were excellent Musical Instrument
speakers. Originally the D's were AP drivers, and once they became popular for MI amplification (around the time of the "F" designation on the model), the "lettered" JBL's became MI drivers, and the SR drivers adopted a numbered identification system. MI drivers sound good
when used in that application; the idea that the rig as well as the instrument make up the "instrument". Guitar players "get it", but some bass player don't. Which is fine, whatever floats one boat, it's music
not rocket science.
I prefer my bass rig to be "musical", and like the majority of guitar players and many pro bass players my rig is an integral part of my tone. I currently gig with a '68 Sunn 200s into a Sonic I-40 cab loaded with a K140. I put it on a X stand at head level with a Heil PR40 stuck in front of it, and use a JBL PRX812W as a floor monitor in front of me. Much easier to transport than my big rigs, sounds absolutely phenomenal, plenty of stage volume. Let the PA do the "heavy lifting".
Fearful cabs are far closer to a small SR cab than a MI cab, Eminence makes a real nice driver, lotta bang for the buck, but is amateur/semi pro grade as far as SR drivers go. The old "get what you pay for" scenario. a 3015LF is <$200, a 2226 is >$500.
The Fearful is an amateur, free ware designed two way SR cab. no reason it wouldn't sound nice, basic WinISD on a driver that Eminence provides amateur design assistance with, crossed over "by the book" with a 6" mid driver. Through another forum it developed a following of unenlightened "internet experts" for a minute, but was never taken seriously in the pro world.
Honestly, if you were looking for a pure unaltered bass guitar to ear sound, just run direct and use a professional monitor, there are myriad pro
designed options available. There are powered 15" 2 way speakers/monitors starting at a few hundred bucks that you can plug straight into, that will absolutely destroy the GK/Fearful if you are looking for full range accuracy and need a free standing rig for your purposes. Heck, you can get a 1500 watt PRX815W for $900 that you can plug straight in to, even control it with an ios or windows device via bluetooth that will carry a FOH many times what that GK/Fearful rig is capable of.
The bottom line is to play what sounds good to you
. The only way you are going to even get close to the SPL
of your GK rig with the Sorado is to put a bunch of efficient drivers with it, maybe 2 X 215 cabs, or mic it and run through the PA. But never in a million years will that transistor amp through a homemade PA speaker cop the tone of the Sorado rig; provided of course that your D140 is healthy.