The Sceptre and the Sentura II heads are identical. The only differences are the names and the cabinets they came with. The D-15S was a rebranded D130, which was used quite a bit in guitar amps at the time. They're not terribly bright sounding - the response is pretty flat, and starts to roll off around 2500 Hz, an octave below most dedicated guitar 12"s - but they are rather efficient, putting out a lot of sound for a given input. That was important, back when 60 watts was a lot of power. The Sentura I had the same preamp, but a smaller power amp, only 40 watts.
Given the similarities of the amps, none of them "need" a 15" to sound full. Sunns are essentially hi-fi amps. They're going to sound pretty good into any speaker, and the resulting sound will depend to a great extent on the cabinet it's going into. The sealed 412 that came with the Sceptre will sound different than the ported 15" that came with the Senturas.
+1
I think the tone of those AlNiCo JBL 15's was fashionable for guitar in that era, and as Isaac points out it wasn't just Sunn; the 15" JBL Sunn used was the D130
F, F as in "Fender", and was used very extensively by numerous manufacturers in the 60's for guitar.
Boost and Distort came along with the solid state Concert series amps. I don't recall ever seeing an earlier footswitch, but Sentura and Sceptre heads don't have boost or distort functions, just reverb and tremelo.
My Sceptre's OEM pedal just says Reverb and Tremolo, I'm pretty sure you are right about that pedal modification coming along after the introduction of the transistor Concert series
As far as the catalog descriptions of "exciting sound", "driving sound", "outstanding sound", etc...
That's simply creative marketing palaver, quite possibly written by someone who never (or couldn't) even
played the amps they described like that.
Just like all the different "amps"; they were really just different names used to describe identical amps with different speaker configurations.
In that era Sunn basically made
6 MI amps. 40, 60 and 120 watters, one of each with reverb and trem - one of each without. "Bass" amps were the "without" version with slightly different tone shaping configurations.
They had
12 different names and individual catalog descriptions though.
The "PA" series had an additional gain stage because they were meant to have microphones plugged into them, but were essentially the same other than that.