How far is your SUT from your Phono Pre/Preamp?

@dhcod ​​​​​​the good news is that if the Jensen's inside the box can be quiet there is no reason an external sut cannot be too. Between grounding, and in a worst case scenario additional shielding, you should be able to get the noise down...

I use a .5 meter Atlas Elements Integra phono specific cable that has very low capacitance between the SUT and the phono preamp. Also, I do Not ground my SUT, as the turntable is already grounded to the phono preamp. Adding too many grounds will add noise. That is what I have found in my experimentation. Of course, all systems are different and I'd advise to do what works.

Up above in this thread, someone suggested experimenting with the physical orientation of your external SUT in relation to the power transformer of your Lamm phono stage. In addition to playing with grounding, have you tried that yet? There is a possibility that will make a difference. Also Jensen Transformers are very very good. I am surprised you find them to be inferior to even a low priced external SUT. Perhaps Lamm does not use the top of the line transformers available from Jensen. You might consider replacing the Jensens in your unit with the best current Jensen SUT.

Interesting. I had a similar problem with an SUT, (not a powered one) as previously referenced by another. Check out your turntable cartridge leads and ground. Also the level or gain if any at the preamp. That was my problem. Too much gain at preamp. Hope this helps. Good luck, these issues can be maddening. 

@dhcod I have real trouble believing that the Jensens are actually worse than your outboard SUT. But in order for them to work right, the Jensens must be loaded correctly at their output. I expect Lamm has a provision for that. The loading changes from cartridge to cartridge and Jensen has published a pdf on their website with the correct loading values based on your cartridge.

It may well be a power transformer associated with the SUT that is causing the hum. IOW look at associated equipment to see if there is a power transformer nearby that might be causing this.