I had a set of the Imperial 7G. Woofers had cloth surrounds and the cones were a blue color like the mids.
one 3 1/2" mid-range in each and one 12" woofer in each.
Both mids needed to be re-foamed, which I did; those 3 1/2" foams for these were hard to find at the time.
The Imperial 9's in the OP have had all 8 mids done for you already as well as the woofers, and maybe put new binding posts on the back (they don't look original). All that's a big plus.
Initially I really liked the Imperial 7's, but then I got a set of Pioneer CS-A500 which are a sealed acoustic suspension with tighter bass.
The Imperial 7's were on the boomy side, being bass reflex design. I read somewhere that you could get Marantz 'Vari-Q Port Plugs' to tighten up the bass.The Marantz 9 are totl and quite desirable to Marantz people
in my opnion
My Imperial 7 grills were missing also, a common problem, as they were a geometric cube foam design that disintegrated faster than the mid-range surrounds over the years. I wonder if the Imperial 9's had those foam grills also.
I also read somewhere that the Imperial 7's were marketed as a sort of poor man's JBL L-100 Century, but I don't know if they compare favorably.