There is always something that one loses out on when one moves within a line. Sometimes the loss is actually a gain of its own. Then, any other gain is icing and cherries and all that.
Back in the day, the SR200s or 300s were beyond compare until the HP-1000s hit the market. Then, the positives of the plastic models were augmented by the alloys, and other new positive attributes were also present making these cans legends if only within that lineage.
The same can be said going from the SR60s through the older RS-1s and then the PS-1 and GS-1000 and finally the PS-1000 before the 'improved' models hit and of course now the addition of the PS-500 which may well be the HF-2 with a bit of a different fit and finish.
Alessandros can be lumped into the above groups too. Grados, perhaps more than any other series of headphones, reproduce consistent qualities between models that, when superficially compared, provide with a familiar result: the house sound. Despite the similarities, there are differences that, for some, are enough to keep shelling out cash for the benefits, sometimes only in the minutia. Is this what you are after?
HD800s, HE-5s, HE-500s, HE-6s, T1s, R10s, SR007s, SR009s, LCD2s and 3s and so on, they all have their qualities, all of them do wonderfully well at reproducing sounds fed to them. Some perform far more handsomely at reproducing the essence of music. Of course, this isn't decidedly objective and hence the fun that are audio forums
Are you able to list, perhaps in certain specific yet distinct categories, areas you want improved upon, changed altogether slightly or radically?