Any PM, upgrade or user advice for the PAR-124a and 116 mod.?

I just invested in a Princeton Applied Research PAR-124a Lock-In Amplifier that came with the 116 Differential Pre-Amplifier module.

Before I turn this on, I want to read into more the two manuals I found online as well as try to find others with user experience regarding use, preventative maintenance (PM), modifications, improvements and/or custom upgrades/options.

Any insight into use of the system is kindly appreciated.

My plans are primarily for ULF/ELF/VLF radio signals monitoring, logging and most likely DSP as I plan to couple an interface to the readout dial meter at the least. I might be overthinking what I need to do as I do see there are outputs… I was offered a discounted price and accepted the offer as I plan to work with later this summer and most likely this fall. Fun with other projects for the time being. :-|)