I am no expert and I think one of the Bottlehead crew would be much better at helping here. I also don't have the Speedball installed and don't know how this will affect your resistances. However, I can note that with my Crack there is no hum. In both the original and rebuilt Crack, I did not lengthen any of my wires. All of them were replaced with the appropriate length of wire from the leftover wire I had from the original build.
Assuming that this relates purely to the insertion of the headphone jack, have you reflowed the solder joints you worked on? Are all of the places you lengthened the wire well insulated and not grounding to the chassis? I may be completely off-track here, but I believe it is crucial that there is only one point to ground (RCA jacks in the case of the Crack).
Also, have you checked the ground path (
http://bottlehead.com/smf/index.php?topic=4812.0) - I would start off with the areas you worked on. Usually you would have 2.49Kohm of resistance measuring from T6/T10. Don't think you can access those with the Speedball installed, but could you measure those resistances elsewhere?
That is really all I can think of, and if that does not indicate any problems, then the experts would need to assist.
