What is the difference between the FeQ-50 and EQF-100?

The EQF-100 uses Jensen 990 op amps and vacuum tubes for make-up gain while the FeQ-50 uses a quad op amp pushing no current with a discrete transistor buffer for current gain. The EQF-100 has all custom inductors and the FeQ-50 has one custom inductor (low frequency) and the rest are off-the-shelf parts. The EQF-100 has more frequencies available which is directly related to cost as each frequency selection utilizes an entire LC circuit. Other differences include the size (with the EQF-100's increase in size is a direct increase in cost), the fact that the EQF-100 has variable bandwidth (Q) and also includes switchable high-pass and low-pass filters.