Was looking for sdc tube microphones, and these catched my attention because of very natural and lively sounding, which reminds a very old Neumann sdc tube microphones. SU-013 tube can be used almost on every source, especially if you like a very smooth and musical high frequencies. These are similar to SU-013 FET but has a signature tube sound depth and color. Well, one thing to keep in mind that it has noise floor: > 18dB (A-weighted), if for some very quiet sources such noise level is too high, then better is to look for FET sdc microphones, tube microphones almost always has higher noise level than FET mics.