Although I have not tried some of the others mentioned above, the Electraglide works wonders on my Joule preamp. Also improved my [highly modded] CD player (thought not as much - ended up using Techline on it), but neither worked as well on solid state amps - using TG Audio there with great results. I'd therefore recommend Electraglide on the preamp, something a little cheaper (but made for front end pieces) on the digital. Don't have any experience with cords for tube amps so I won't comment. You're on the right track - big improvements changing cords!
Power Snake Mamba vs. Synergistic Master
Anyone had a chance to compare the Mamba vs the Master Coupler head to head? Opinions?