Hi Spank,
The Glentek Amps should accept a +/-10V input which is the industry standard. I found this in the manual:
4.4.3 SIGNAL INPUT:
The GA370 has two signal input configurations controlled by JP1. Configuration A (JP1 set on A) has two
single-ended inputs SIG. IN. (J1-2) and AUX. IN. (J1-1). Configuration B (JP1 set on B) has one single-ended
input AUX. IN (J1-1) and one differential input DIFF. IN (J2-1), DIFF. RET. (J2-2). (Refer to Appendix B,
drawing 370-3003). Typically when operating in the velocity mode, the input signal range is ±10 VDC. The
input voltage is summed with a precision DC tachometer to provide accurate velocity control at the servo motor
shaft (see fig. 3.1). The Signal Gain potentiometer RV2 (20 K), and Auxiliary Gain potentiometer RV1 (20 K),
adjust the motor velocity desired for a given input voltage velocity command (see 4.2 for recommended wire
type & size).
You might try this simple test.
HTH