I finally listened to some FLACs on my AWE64 and Audiotrix 3D/XG through the Rolls HE18 units . Subjectively, I do not notice any difference with or without the HE18, except for the lack of hum .
One curious, but harmless, side effect I was not expecting is that I had to re-adjust the gain on the DPS12 mixer for the volume of wave out to be the matched for both cards . When running through the HE18, output levels seems more evenly matched than without and thus gain settings on the mixer are now essentially identical for both cards . This is unless I had previously done a crappy job of adjusting gain and had not noticed until now. I am not going to unplug anything to test . Could this be due to the HE18 compensating for differing output impedances of the sound cards' line outs ?
Tests/adjustments with the Gravis Ultrasound, SC-88, etc will have to wait , but the hum is gone there too (HE18 between component switcher and DSP12).
I decided to think long-term and, since I already had everything except the cables to go balanced, I bought what I needed and installed it . I did not need to, but it's one extra level of protection against noise/interference, so why not .
So my hum issues are essentially resolved for now, short of buying a new mixer/multi-track recorder with phantom power and active DI boxes or getting passive DI Boxes with something like Jensen audio transformers (at 200+ dollars a pop, that's quite unlikely to happen) .
Thank you to everyone for their input. Hopefully this thread will be useful to others in the future .