Ground Loops can be problematic. You are going about it more-or-less correctly (work your way back from output to input). Typically there is a cable or AC line that is part of the problem (forms one leg of a loop). Sometimes you can't really remedy the situation if the issue boils down to the internal ground structure of a component.
Member audioengr makes a good post with the troubleshooting procedure outlined comprehensively.
However, there is one sure-fure way to fix your issue, should you run out of hair to pull out. Use an isolation transformer of sufficient quality to be audio grade. I recommend Jensen transformers ... if you're handy you can construct a box yourself with your choice of connectors, etc (they are passive devices, so relatively simple to deal with ... choose the transformer wisely and keep leads short).
Alternately a ready-made solution with the correct transformers can be purchased. For your application, a Jensen Iso-Max isolation transformer placed between the preamp and power amp should eliminate all hum issues. The Jensen products are of high quality physically and sonically.
Be sure to purchase from a vendor that accepts returns, should you find you need one between all your sources and the preamp (unlikely, but not beyond the set of possibilities ... ground loops are unique and system dependent). In that case it might be worthwhile to explore cable substitutions, and assess the results vs the Iso-Max solution, or a combination. There is also the possibility that you may find a sonic penalty you can't live with, although I would say in my experience that isn't usually a huge issue provided you are dealing with a quality transformer manufacturer and an appropriate version.
Having said that, I am quite confident the one unit between the pre and power will work in almost all cases. Jensen make a number of variants (mono, stereo, audio, video, RCA, XLR, 3.5 and 2.5 phone, etc). You probably want the CI-2RR model. About $US 150, and note that they are sold to both the Professional and Consumer markets, so you can price check with Music Stores as well as Audio or A/V vendors.
Isolation Transformers work because a transformer can be constructed to pass AC of specific frequencies but inherently block DC. Although AC hum is at line frequency and various intermodulations and harmonics of that frequency, the connection that is the root cause of the ground loop is a DC loop.
There are those who are suspicious or outright dismissive of transformers in the audio chain. The takeaway is a given transformer requires high quality copper (typically, could be silver or gold for that matter, but ... well ... ) and other metals, which cost money in a fairly rigid and unavoidable way. In other words $$$ = quality and performance, and there isn't much else to add in that regard. They are what they are, and you most certainly get what you pay for, no way around it (for new construction).
Not sure if it's useful to you, but it's worth mentioning: optical cables break ground loops, so that is also an option with digital interconnections.http://www.jensen-transformers.com/product/ci-2rr/