That's an old post. I would suggest using a buffer although I don't recall it being required in the 990.
KJ7GHK, what software are you currently using? You could try downloading the latest version of HDSDR. This is what you'll probably use for your panadapter anyway.
Download the latest version (2.80) here:
https://www.hdsdr.de/In the options settings of HDSDR, there is a tab called RF Front End or something like that. For normal listening you have to make sure the "external antenna" is selected. When using it as a panadapter, you have to select "IF" and input the first IF frequency of the 990, which is 47.210 MHz. There is some tweaking to be done, but this will (should) get you a working panadapter.
Once you have determined the RSP1A is working, I can help you with the panadapter portion.
Max