[Francis Vaughan]

The Lehle is a passive splitter, despite all the powered stuff around it - the actual signal path would appear to be simply multiple secondary transformer with a few relays to effect the switching.

Both the Palmer and LittleLabs splitters are active - they include a buffer amplifier as well as transformer isolation.

I think the previous suggestions have all been predicated on the assumption that your hum problem is a ground loop. (Which is exactly what I though as soon as I read the initial post.) Thus the concentration on transformer isolation.

It sounds more as if you are simply picking up some residual hum in the system from any one of a number of sources. Does the Lehle hum if you run it with no power? It seems that it will default to splitting the signal to both outputs if not powered. That would tell you if there is hum injection via the power supply (it won't be from the usual causes of power supply, but it might capacitively couple enough junk to the chassis of the unit to be an issue. What about if you only connect one amp? What if both amps are connected but no guitar?