The X-1 is not an overall warm preamp, it is neutral sounding to my ears. The best power cord I tried was the Elrod Sig 3 on it. It added a slightly warmer mid range with a little more body. I never felt the bass was less than solid in fact I found it to be one of the best bass preamps I have heard. Try a 14" bicycle inner tube under the unit to see if it makes a change in your system and if it does and you like it, you'll know what to do. Also put some wood blocks under the unit and experiment. My experience may be quite different that yours do to room and vibration effects. I never really heard any real difference with the power supply on top, under or separated from the main unit but there are others who will have a different experience. I compared the X-1 to over a dozen preamps including tube preamps. Each preamp had their own strengths and weaknesses and added a different flavor to the sound but the only one to make me want to change was the Tom Evans Vibe. It was that much better and more $$$$. I never found AC a problem with the X-1 so maybe someone else can offer some suggestions.
Happy Listening.
Happy Listening.