Maybe this isnt what you are looking for, but if you look long enough on ebay you can find a preamplifier for a lock in. These are usually made by stanford research or princeton applied research (eg&g PARc).
These often have a 1 to 10 MEGohm full differential input, and will really bring a rather VLF deaf receiver to life, as in many cases the poor performance of some receivers in that range is just because of their lack of sensitivity.
Beware bandwidth limitations though, some of these have an upper frequency of a kilohertz or less. many, however, will go up to a couple hundred kilohertz.
PS. there are also transimpedance models that have near zero input impedance, and a gain between 10 KV per amp or 1GV per amp. these are good for loops