Hi Nick,
You need to be sure on the card you have. Seems like according to your description, your 'tron has the newer card (not the CMC-10, that's the older one) or the upgrade SMS-3 or SMS-5 from Streetly or Markus. Like I said in the previous post, the newer card (or Streetly or Markus upgrade cards) have the upper rail leg of what's shown on the schematic (SKT 1, 5 and 8 ) on the card itself, not in the PSU. That may be why your PSU has been modified (and as you indicated). The original PSU had 5 big caps under the housing, the newer/upgrade card reduces this in the PSU to only two caps necessary as the others are, again, on the card. If the procedure was done right, there should be no hum. However, if the caps themselves were never replaced and are still the originals (at this point over 30 years old), they themselves could be the reason for the hum.
Unless you've previously experienced this 'tron having no hum, know that M400s are prone to hum relative to where they sit in relation to RF and other hash. Their 35 heads may also have ties to hum, too.
I don't know of any Streetly mods for output trannies on the M400.
For all of these issues and more, I would call Martin at Streetly and discuss with him to be sure. He's a good lad and will set you straight. Tell him Chris sent ya, I placed a big enough order with him last year; he should know me pretty well by this point! =)
Cheers