The MP-1 alone has very little hum. It has the DMRT, noise mod and I just replaced all the plate and grid resistors to 1/2w metal film on the tube board. I can only hear it when I crank the power amp fully and it's barely noticeable under the normal hiss. OD1 3.8, OD2 6.0, master 5.0, guitar unplugged. I never tried to change the tubes though, still has the GrooveTubes 12ax7 in it.
The SE-70s alone have more hum. When I take them outside the rack the hum doesn't change in intensity. And I mean only those connected to the power amp and nothing else.
The power amp by itself, when cranked at full volume, has a tiny bit of hum but you hear it only with your head against the speaker. As soon as I connect the SE-70 output to it the hum increases. its's still quiet but it's more.
When I add the MP-1 to the chain I get a slight further increase of hum. So today I insulated the rack ears on all the units with electrical tape and insulated the screws with plastic washers and heatshrink tubing. The situation did not change, but I added a ART hum eliminator between the MP-1 and the SE-70 which fixed the increase in hum, so now I am only left with the hum generated by the SE-70 themselves.
The SE-70s are powered with wallwarts. The adapters have a ground pin but at the other end the barrel connector has only 2 contacts, tip and sleeve. So I wonder how they get grounded power wise, is the whole circuit floating? The transformer I am using to power both in the rack is a 240v triad electronics toroidal transformer, with 2x 12v taps, 2A each tap, it's bolted to a rack tray, does the job.
Both the SE-70s get quite hot during operation and the have a bunch of electrolytic caps in them, so maybe they got old due to the temperature. I wonder if replacing at least the ones around the voltage regulators and power supply could make a difference in the hum.
EDIT: I did try to put the ART Hum Eliminator between the SE-70s and power amp, but that didn't remove any hum from the SE-70.