I've recently installed a second digital cable receiver and the signals are a little bit off now.

Original connection:
main cable -> splitter -> cable modem
main cable -> splitter -> digital cable receiver

New connection:
main cable -> splitter -> cable modem
main cable -> splitter -> splitter -> digital cable receiver #1
main cable -> splitter -> splitter -> digital cable receiver #2


On my receiver #1 before making the changes my signals were this:
FDC RX -11 @ -8
RDC TX 47


Now receiver #1 has this:
FDC RX -15 @ -13
RDC TX 50

And receiver #2 has this:
FDC RX -17 @ -16
RDC TX 53

My FDC is on 72.75Mhz and RDC is on 19Mhz
What kind of amplifier would I need that would bring the above values to normal?

Has the picture been affected at all? You are too way down into the detail weeds - only worry about signal issues if your display has been negatively affected. Plus, if it is a signal strength issue your cable co should be able to address that.

The picture QAM's are fine. Signals are within norms and the picture is good.

It's only the FDC that is suffering and that's the channel through which the guide's info is sent. I may have some issues in the future with some guide info missing or missed OS updates.

As for RDC, it's a bit near the 55 dBmV limit, though I'm not sure what happens when it's out of norm heh :)