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It seems that you have do have different schematics to the one that is
in my SP8 manual. I looked at the one on the ARCDB and it is from a
later version. From the photos on that website it seems I have a Mark
I preamp. I don't know whether it's revision 2 or 3, but the schematic
does mention changes made in 1981, so I would assume it's not the
original version - called revision 1 on the ARCDB (although how the
first version of something can be a revision, I'm not sure!).

Anyway, my manual does not show this resistor no. 27 in this
position. Rather, resistor 28 is in a similar place to resistor 83
and there is shown a line parallel to it that has a gap in it. The
lines end in circles with crosses in them similar to the ones at
either end of your r27 on the ARCDB site. This is the place where it
says that r29 should go if needed. I also note that the gain
reduction quoted here is 10 dB's rather than the 6dB's mentioned for
my 39.2k resistor.

If you look at the photo of the Mark 1 SP8 internals on ARCDB you can
see the bottom of 2 of the posts I was referring to where they
protrude from the other side of the double sided circuit board.
Frustratingly, there is no photo of the underside of the board & mine
is very different from the underside photo of the later amp that is
shown.

Here's how you find them:
If you locate the yellow Wondercap to the far left, below the mains
transformer, you can see the shiny edge of the heat shield I referred
to. Now, between the Wondercap and the heatshield there are 4 solder
tracks that go under the cap. The left hand one goes under the left
hand end of the cap, leaving 3 more going under the upper edge of
it. The track to the left and the one to the right show the bottoms
of these posts as shiny discs. I think r29 should be soldered across
these posts because I measured the resistance between them and found
39.2, which I assume to be the value of the paralled resistor, r28.
If you then look to the right of these posts, there is a black
resistor with its left hand wire going into a spot of track with no
begining or end - I think this is r28.
If you want to see what the posts look like from above there are two
more over near the top of the amp between the topmost psu capacitor
(one of the upright ones) and the transistor mounted in the heatsink.
There is a black device soldered between these posts - not sure if
it's a resistor or not. The lower of the two posts is easier to make
out than the other one.

I wonder if anyone looking at this has a similar vintage SP8 with a
resistor across these posts on their amp?

Cheers
Chris