Last (probably) question :

My cable runs being no longer than 6 feet each, would it be worth getting the cables for balanced transmission ?

In other words, if running unbalanced through the Rolls HE18 seems to solve my hum issue, would there be a point to going balanced for a 6 foot cable run between the Rolls HE18 and the mixer (DPS12) ?

I assume the answer would be no, but I would like some thoughts from people who actually know what they are doing in the audio field (as opposed to me) .

To be clear, as Tiido wrote, from a hum reduction perspective, I know nothing would change between balanced and unbalanced as I would still be running through transformers for the ground lift .

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 .

Just an interesting (IMHO) update . I recently realized that only my AWE64 still has a hum-like noise and only when I am using the Voodoo 3 through the OSSC . None of my other 2 sound cards or any of my MIDI modules has this issue . This is in spite of the isolation transformers and balanced audio connections . I initially did not notice the problem because I was running my tests with the Geforce FX5900 .

Anyway, the cause was easy to identify by a process of elimination : it was the 5V DC power supply that I was using to power the OSSC ! I replaced the adapter with another one and the AWE64's noise disappeared .

Why was only the AWE64 affected by the noise ? Who knows ?

One of my Rolls HE18 died (started humming on it its own, the humming stopped once I bypassed it). I wrote an e-mail to the company and have not heard back in 2 weeks . I am not exactly impressed by the quality of their service .

EDIT : They have a support form that is only to be used by customers in the USA . They also say to contact a distributor in buyer's country for support if not located in the USA . I purchased from an American company (B&H) and am located in Canada . Additionally, there is no list of distributors (US or otherwise) on the WEB site .
In other words, if you are not located in the USA and you have an issue, good luck . I honestly expected more from Rolls .

EDIT2 : I sent them a message on Facebook . I wonder if they will respond .

EDIT3 : Rolls did respond on Facebook and referred me to my place of purchase . I will be contacting B&H .