I've owned several of the original* super radios, have used 1,  of the super radio 2s, and owned a super radio 3.
For Basic sensitivity, selectivity and sound, IMHO,  the 1 is overall_ the best.
_However, the 1, of course is only_ Mono, 
also, it tends to "smush the AM audio into its' narrower "Passbanded AM carrier.
of course you can tune the carrier up on the side a little, and use the tone controls to advantage.
the AFC on the 1 is very good, although when on, you won't get many of the weaker signals between stronger ones, as the AFC really does home to the stronger signals and keep the radio almost locked on them.
***The weakest link in the 1s, is the easily breaking dial cord! has been a problem with several I know of.

the 2, if memory serves, gives FM stereo via phones,
is not quite as selective on AM, though pretty adequate, and because of its' seemingly lesser narrowness on AM,  the AM audio sounds a little less smushed,
I think the AFC on the two is a little more forgiving, but it's been a long time since I played with one.  I understand the dial cord also has been a problem on the 2.
Meanwhile, on the three,its' AM sensitivity is pretty good, although not as good as on the earlier two models.
On A M The radio seems to have more sound fidelity especially with the wide narrow switch set to wide, than on the earlier two models, but apparently they chose clear wide ranging sound on what was tuned clearly, over sharp selectivity.
For Even with the wide narrow control set to narrow, if there are several fairly strong signals fairly close together on the dial, there is little available separation of stations, and the radio is swamped, overloaded by the strongest signal or signals, with the other weaker signals somewhat audible in the background with the stronger signals overpowering them.
Even with the wide narrow switch set to narrow, the selectivity, and carrier narrowness,  is much less pronounced, than on the earlier two models.
Because of the inate wideness of the radios' characteristics, the AM stations you can dial in clearly, on the 3, seem to come across with more "Full Spectramed Audio" than on the earlier two models, but to me the sacrifice of selectivity for sound quality is unacceptable, and IMHO, the FM semsitivity and selectivity on the 3, is much much less than observed on the earlier two models.
I'm not sure what to tell you to do about finding someone to restring dial cords, or why that seems to be such a particularly prevalent problem in such an otherwise, over the top, receiver.  But it is possible to somehow cut into the case of the radio and tune the variable capacitor directly rather than fooling with the ddial cord.  I know, that ruins the resale value of the receiver, but it's a thought.  (don't know anyone who's done that on a super radio, but on others?  Yes.
*Bottom line, for real radio listening, on either AM or FM, if you can find a super radio two, and or for sure,  an original model, at a decent price, / (I've found them sometimes for as little as five dollars, but also run across them for much much more;)Truth is, that for An analog Receiver, either is a real cut above many other analog or digital units out there:
HTH, Danny Dyer, Wb4idu.