Has anyone run a MTH Adams or Jordan Water Tender with their Lionel Legacy engines?

That si only when you tether it to your MTH DCS loco. It has a plug on the front of the tender to run the reverse light and possible an electro coupler though I am not quite sure if there is an electrocoupler on these tenders. If you run it behind a Lionel loco you have no way to connect it to the loco so its really not an issue. Go ahead and run them you wont have nay issues.

FWIW, MTH will not honor warranties on eBay purchases since "eBay is not an authorized re-seller" so if you're buying from eBay make sure they are an authorized re-seller and get an actual store sales invoice.

This is right out of the manual

CAUTION: This Auxiliary Tender can only be

electrically connected to an MTH Steam equipped

with ProtoSound 3.0 Electronics. Making an electrical

connection to any other version will damage the

components.

Might that be due to MTH using LED's in the Protosound 3.0 engines versus using incandescent bulbs in Protosound 2.0 engines and a PS2 engine would pass along too much voltage?

An LED can usually handle voltages between 1.5 and 12V so the in this case the voltage is not what will burn out the LED, it will be the current. You could run this with a PS2 loco provided you install a current limiting resistor on the  LED. But that wasn't the OPs question

Thank you for the information on the MTH Adams and Jordan Water Tenders. I have the two Lionel Union Pacific 907353 and 907356 for the Big Boys. I missed the Lionel Adams Water Tender that is sold out.  I figured this pair would look good behind the black UP #844. The backup light is not an issue as long as the water tender is not creating any interference with the engines. If possible, I will tuck the wire for the PS3.0 engine into the tender.