Help Hum Loop Noise Issue Problem

[Bassmec]

OK folks I have two 4 X KT 88 all tube amps stacked each amp is eq'd to match the cabs it drives. AT the moment I am using as bss DI Box from my bass feeding via the direct socket to one amp and the buffered socket to the other.
Each amp is unusually quiet noise wise having got very good tubes and bypass caps throughout the upgraded power supplies.
When I plug the system together and play through one amp with the other on standby its fine noise wise. If I switch the standby on on the other amp I get a noticeable 100 ish Hz hum from both amps.
If you pull the cables going to the amps out of the DI box and you touch the two jack sleeve's of the cables from the amp inputs together
You get this same hum. Pull them apart both amps just hiss a little.
Both amps are powered from the same four way well earthed plug board.
Will a transformer based isolation system cure this problem and will I have to get a transformer made if so what is the ideal impedance
for my bass which has a gibson humbucker bass pickup off a grabber
and a pre cbs fender jazz bass pickup in parallel it measures 50K ohm
resistance at the socket as I use it.
Or is there a simple isolation box made for this job.
Why is this happening?. buffering should cure this but it just lessens the hum in relation to just wiring up a y cable from the bass.
Is there any milage in trying swapping the polarity of one of the amps mains transformers in the amps power socket.
Having two amps stacked is common what's the trick to this?.
I know its not a big hum but it pisses me right off.

 

[bongomania]

I do think a transformer isolation box would help. Lehle makes a box called the P-Split ( [Invalid or Expired Link Removed] ) which would be just the ticket; it's pricey, but possibly worth it as a ready-to-use option. For cheaper, check out the Jensen transformer website. They have a wide range of xfo's for sale, and they provide (for free, on the site) a broad range of DIY instructions for different xfo projects.

Radial makes the JS-2 ( Invalid Link Removed ) which would do the job but you'd need to use it with your existing DI to convert your bass signal to balanced.

 

[billfitzmaurice]

Why is this happening?. :

Read this:
[Invalid or Expired Link Removed]

Chances are you have the amps grounded together at two points, via the power cords and via the inputs. Simply disconnecting the shield at one end on the input cord to one of the amps will probably fix it.

 

[Bassmec]

Both replies are right I have already tried Bills Idea it works to a very large extent.
but not absolutely completely and I am such a fussy pillock I will ask Sowter transformers to make me another transformer, as I am a firm believer in "Rolls Royce" transformer balance systems. And I have got these amps so quiet on their supplies/tubes ETC and have spent so many man hours on them to get them as good as they can possibly be, "why spoil the ship for a hapeth of tar" as my great uncle used to say.
Thanks again!.
Regards Fairfax