Do you have a Digital Volt Meter (DVM)? If so, some simple measurements can put the ground loop issue to bed. On the Ohms scale, with the equipment unplugged, measure from the ground pin of the power cord to pin 1 of the XLR. What is the reading? Is it the same as the ground of the RCA connection? I would do this for the preamp and amp for starters.
BTW we make a tube preamp called the MP-3 that is fully balanced, has dual outputs and 3 balanced inputs (one of which is phono if the preamp is optioned with a phono section, which is also balanced). You might be able to find a used one or a demo at your price point.