That seems like an awfully low threshold for a preamp to overload. Why add more attenuators, cables, connections in line with the existing preamp's attenuators just to pamper a preamp that already contains switches and/or potentiometers?
I don't have experience with the Lampi Atlantic, but my Golden Gate has a similar output level, actually higher at 5v. I auditioned a Golden Gate with the built-in attenuator and source switching. At attenuation down to about -10 dB they were very close, but at more attenuation, the Tortuga was superior. The Tortuga doesn't lose dynamics or low level detail with increasing attenuation. Channel separation is superb, better than any active preamp in my experience, and the bass remains as powerful and articulated as at full volume.
The key to the Tortuga is that there are no switches or potentiometers (or capacitors for that matter) in the signal path. Even the input source switching is done by LDRs. Plus, you get the benefit of input impedance control from the remote and in the LDRxB, the opportunity to have absolute phase/polarity control with one click from the remote. The computer controlled attenuation settings can be re-calibrated at any time (again from the remote), so any long term drift of the LDR's attenuation is a non-issue. I usually recalibrate mine every few months or so, but then I tend to be obsessive about my system.
Seems to me to be time to move on.