Since you don't have the problem using the same mics on your Tascam it's obviously not a mic problem.
Basically for the mic inputs you've got Normal or Low gain settings, phantom power on/off, limiters (engaged by factory default), high pass filters. Also meter ballistics are assignable and the audio output can be attenuated by up to 40dB in 1dB increments.
Perhaps what you are experiencing is related to an output attenuation setting. Especially since you bought it used if you can't get it resolved by toggling settings it would be advisable to send it in to Sound Devices for a full checkout and/or repairs and calibration.
Some info from the manual from manual below that may help...
Microphone Level Control
Microphone gain is controlled by the front panel recessed knobs. The gain control adjusts an analog gain stage and functions similarly to the input trim on a mixing console or stand-alone microphone preamplifier. Gain is controllable over two ranges, normal and low.
Gain Range (microphone-level only)
The microphone inputs operate in four gain ranges, NORMAL, NORMAL FADES TO OFF, LOW, and LOW FADES TO OFF. The NORMAL range controls input gain from 24.3 dB to 67.4 dB of gain. The LOW range controls input gain from 9.3 dB to 52.4 dB. The LOW ranges are useful for high SPL recording environments. The fade to off options allow for fader-like control of your gain. NORMAL FADES TO OFF provides a gain range of off or 0 to 67.4 dB and LOW FADES TO OFF pro- vides a gain range of off or 0 to 52.4 dB.
Output Meter
The meter uses energy efficient LEDs viewable in full sunlight. The 702 output meter is unaffected by shock or extremes in temperature and humidity. Meter ballistics are Setup Menu selectable among VU, Peak, Peak-Hold, VU + Peak and VU + PeakHold.
The meter uses a compound metering scale which increases meter resolution in the most important part of the scale. From −50 to −40 dBFS, each LED segment equals approximately 10 dB. From −40 to −12 dBFS, each segment equals 2 dB. From −12 to 0 dBFS each segment equals 4 dB.
Analog Line Out L, R
The analog line outputs are active-balanced line-level signals on Switchcraft TA3M locking connec- tors. The output level is a nominally 0 dBu at −20 dBFS. The output level can be attenuated from the Setup Menu by up to 40 dB in 1 dB increments. Both left and right outputs are attenuated equally.
Meter Ballistics
The output meter can be set to display any of five types of meter ballistics: VU, Peak, Peak-hold, a combination of VU and Peak, and a combination of VU with Peak-hold. The meter ballistics are selected in the Setup Menu.
VU - (Volume Units)
Ballistics correspond closely to how the human ear perceives loudness and provides a good visual indication of how loud a signal will be. In VU mode, the attack and decay of the meter signal is 300 mS. While giving a very good visual indication of perceived loudness, VU meters gives poor information on actual signal peaks and are virtually useless for tracking to the 702. In VU mode, the front panel meter labeling is in volume units.
Peak
Peak-reading ballistics correspond to actual signal maximums, but don’t necessarily correspond to perceived signal loudness. A peak meter has a near-instantaneous attack to display maximum signal amplitude and a slow decay to allow the user to see them. Peak metering is essential for digital re- cording, since signal overload can cause immediate distortion. The peak meters front panel markings are calibrated in dBFS, decibels relative to full-scale digital signal.
Peak Hold
Essentially the same as Peak metering where the peak level indication will hold for the peak level indication for several seconds. Peak-hold indicators are useful for metering in applications when an overload condition is unacceptable.
Peak/VU
The meter can simultaneously display VU and Peak level information. In this mode the perceived loudness (VU) is displayed on a bar graph, and the Peak signal on a dot above the VU. With this combination the user gets the best of both VU and Peak metering by seeing both the “loudness” of the signal and the peaks at the same time. Peak/VU is the factory default.
Peak Hold/VU
Similar to VU/Peak mode, this mode holds the peak level indication for several seconds before releasing. Peak Hold indicators are useful for metering in applications when an overload condition is unacceptable.
Output Attenuation
Analog Output Bus
Analog Tape Output
The tape output connection is stereo, unbalanced consumer output level (–10 dBV) on a TRS 3.5 mm connector. Output attenuation affects this output level.
Digital Output Bus
Just as with the Analog Output Bus, the Digital Output Bus (Bus 2) can be assigned signal sources from inputs or tracks. Sources assigned to the Digital Output Bus are exclusive and do not affect the assignments to the Analog Output Bus or headphone assignments. The same signal sources available for the Analog Output Bus are available for Digital Output Bus (see chart above).
The Digital Output Bus appears solely on the AES3id BNC output connector. The unbalanced AES3id output is directly compatible with most S/PDIF inputs.
The format for the AES3id output is selectable between professional AES and SPDIF. In either case the SCMS bit is not set.
The maximum output level is 0 dBFS and can be attenuated in the Setup Menu in 1 dB increments by 40 dB.