Last summer I borrowed an Aleph 3 which has an input imedance of 23K Ohm single ended only and had a some difficulty pushing it with Blue Circle BC21 tube preamp.
I greatly enjoyed the sound of the Aleph mated to the BC21 but its inability to crank prevented me from buying it. I felt it did acoustic guitars better than any of the other amps I was comparing it to.
I believe the input impedance of the Aleph 5 is 10K Ohms unbalanced and 25K Ohms balanced. So you'll probably have varying results depending on which preamp you try and whether it is balanced or single ended.
I greatly enjoyed the sound of the Aleph mated to the BC21 but its inability to crank prevented me from buying it. I felt it did acoustic guitars better than any of the other amps I was comparing it to.
I believe the input impedance of the Aleph 5 is 10K Ohms unbalanced and 25K Ohms balanced. So you'll probably have varying results depending on which preamp you try and whether it is balanced or single ended.