That is a very fine looking cock bird. Nice and broad with that rich mahogany color.
From what I understand the Speckled Sussex has been used in the Jubilee Orpington in the past and also some American breeders used the SS to create the Jubilee Orp here before it was widely available.
What I've read is that the columbian gene is responsible for the black (or blue, etc) color next to the mottled tip. Without columbian the chicken is just mottled. That's why I suspect your pullets with less black/blue markings are incomplete columbian.
Your suggestion about the E-locus got me curious and I went and did a little research on it. It sounds like the e^b birds show better black banding than the E^WH birds.
With a little luck his offspring shows that same groundcolor and featherpattern.
Not that I haven't seen great looking jubilee Orpingtons here, but the marking on the speckled Sussex looks more neat/refined than on the Orpingtons (and Wyandotte bantams ).
Still putting together how to get that same pattern on the Wyandottes.
Three years ago I noticed that the chicks with stripes in the down became better birds when it came to the markings. The total picture was more black and less white. In my first jubilee Wyandotte bantams (2012), the white and the black was very chaotic.
This was one of the first jubilees here. Her down as a chick was very yellow-ish (wheaten?). I decided not to put her in a breeding pen.
This hen came from the same batch and her down as a chick showed stripes. (e^b?)
Since 2012 I have been selecting very strict on the amount of white and always used a cockerel that showed stripes in his down.
So far, by only using birds from my starting flock + offspring, this was the best pullet I have bred (as for my opinion).
Two years ago I started another line by adding two hens from Holland to the flock. The groundcolor is less mahogany, but the marking is improving.
I believe the columbian gene is responsible for way more than only the black band.
Underneath on the left "jubilee" with co/co (gold should be red), right jubilee with Co/Co.