Get yourself a circuit called 4013 like this one: 
http://www.elfa.se/elfa-bin/dyndok.pl?dok=2013017.htm (swedish site, but 
it's a standard component and not expensive). I remember buying three when 
making my mod and still having two spares left.  
 
Viewed from the top (legs down) you count the legs counter-clockwise with 
number 1 being to the left of the half-circle at one end of the IC. Like 
this: 
 
1 U 14 
2 13 
3 12 
4 11 
5 10 
6 9 
7 8 
 
Sometimes the 1-leg is marked with a dot as well... 
 
Attach the IC to a piece of veroboard or simply a piece of thick paper and 
make holes for all the legs. 
 
Solder like this: 
 
1-6 - no connection 
7 - connect to 8 and 10 
8 - connect to 7 and 10 and a wire, 30 cm will do just fine 
9 - connect to 12 
10 - connect to 7 and 8 
11 - connect to wire, 30 cm will do just fine 
12 - connect to 9 and a wire, 30 cm will do just fine 
13 - no connection 
14 - connect to wire, 30 cm will do just fine 
 
Now you should have an IC with four loose wires. I'll call them wire 8, 11, 
12 and 14 from now on. 
 
Open the lid of the SX-1000 and flip up the panel. I'll describe the places 
to attach the wires as seen from the players view with the lid tipted up. 
 
Connect the wires like this: 
 
8 - goes to 0 (zero) volts on the SX-1000, left leg on the PW switch (second 
from left in the second row of potentiometers and switches). 
11 - goes to leg 9 on the 4013 that is part of the SX-1000's own design. 
Find between the octave switch and the vibrato potentiometer (in the bottom 
row of pots and switches). This is the only part that can be a little fiddly 
to do... 
12 - goes to the right leg on the noise switch (second from left in the top 
row, this is the off-position normally). 
14 - goes to +12 volts, the right leg of the PW switch (see note on wire 8). 
 
I'll suggest you staple the piee of veroboard or paper to the bottom of the 
SX-1000 (inside) so it cannot damage anything when you close. 
 
Turn down the noise level pot before turning on the SX-1000. Test it with 
white and pink noise first so everything works like it is supposed to. Turn 
down the level again and switch to the off-position on the noise type 
switch. Slowly raise the level and now you should hear a suboscillator 
signal added, a square wave one octave below the normal sound. Don't crank 
it too high 'cause then it drowns out all the original sound. 
 
And no, wired like this you cannot have BOTH suboscillator and noise at the 
same time, but the noise works like it should otherwise.