At the upper end of the scale, 9 and 10 are virtually the same. I would not go below 9 for Gun Crazy because 8 is equivalent to 3.2/4 on Maltin's scale and Gun Crazy is more like 3.5/4 for me which is a bit less than 9.
Gun Crazy is strong in its own ways, and The Big Combo strong in its ways. Gun Crazy has energy and a kind of subversive anarchic thread to it. The Big Combo has better music, better bits of action like the hearing aid scene, and good triangle conflict. But the action flags at times, talk takes over, and the sexual attraction seems to me more feigned. All comments are memories -- I'm due for a rewatch.
From my pre-modern vantage point, I definitely am not looking for either incest or molestation in modern noirs. But when they appear, it's necessary to figure out what they're supposed to mean and why they're there, why the movie makers wanted them there.
I was unimpressed with Gun Crazy the first time I saw it. By the 3rd time I began to think it has a high degree of uncompromising wholeness, and I favor that. The movie takes an unlikely couple by usual standards, makes them seem more likely, and then follows their story from start to finish without introducing anything else that compromises what the story is.
I'll have to reserve comment on The Big Combo until the next time I see it. My comments above already suggest that what I recall is more the elements of which it is made than an overwhelming thematic feeling. There's that suggestive love scene with Peters. There's the presence of Fante and Mingo, who almost steal the picture. There's that beautiful foggy photography.