Not as impressive as the GX-F95 monster but just as desirable... mostly because of the motorized door which hides everything the F91 holds !
Given the Mitsubishi precedence and disguised managing since 1979 through Akai's bank (the Mitsubishi bank) and a CEO which came from Mitsubishi, Akai after 1979 was to look for less expensive machines and mechanical design.
Also a more streamlined production with extensive use of part sharing across lineups which later gave the A&D brand made of Akai and... Diatone.
Mechanically, the GX-F91 is a GX-F71 with an added PLL loop on the main DC DD motor.
That motor was not necessarily magnificent but it was shared by most Akai decks between 1981 and 1984, decks such as the GX-F51 and GX-F31 or the later GX-7, GX-93 and...A&D GX-Z9000 (1987).
The GX-F91, like most of the later GX-77 production was done at AS : Akai Saitama.
Saitama being a city which seems to have had very attractive estate deals for production plants because many brands had one there from the 1960s onward : Sansui, Akai, Micro, CEC and Alpine to name just two industry minors and three OEM majors.
Build quality-wise, all of Akai's output between 1980 and until A&D began can be tagged as "industrial" : nothing here that could be mistaken for the contemporary Sony TC-K777.
Nevertheless, the electronic features were what Akai pushed forward.
Comared to the GX-F71, added to the GX-F91 is a 4-memory auto-tuning so that bias and eq exactly correspond to the actual tape inserted ; the F71 has the function but simplified and with only one memory switch.
The F91 has even more refined Dual-Fet DC SEPP rec/play stages and 70µ PCB tracks.
A dbx license at this price-point probably was too expensive, as was happening at the same time for the GX-747, but the D-Comp function has nothing to do with that : it is a linear meter display mode which includes all the recorded frequencies.
In the same area, both decks have a very useful switchable peak/vu meters with a scale shifted upward for the "peak" setting ; the F91 allows two peak scale "zooms".
The rest of the features are almost equivalent : three Super GX heads, doppel-capstan with closed-loop, auto-tape selector, auto tape/source monitor, Dolby B & C, MPX filter, bias tweaking (on top of the auto-tuning function), counter/realtime counter, counter memory for auto-play/rewind/rec, rec-mute and automatic recognition of 4s blanks functions to "skip" from one track to another, auto-fader etc.
All that all 1981/1983 top cassette recorders were doing for your fiddling pleasure.
The GX-F91 just looked more intriguing because it concealed absolutely everything it held.
Even if pressed to sell more, Akai in Japan advertised for the F91 very little but did it in two-step style :
a double page with the door closed for the spring launch, and three months later, in the summer, with the door opened !