I also have to add an amp before my SLM when making tuner sensitivity tests. Since it is only before the meter and not before the tuner, the added NF of the amp doesn't matter because I'm only measuring the top of the signal. I use a 4-way splitter when I'm comparing two tuners at dropout.
Code:
tuner
variable /
ANT > CM7777 > coax > power > attenuator > splitter
inserter \
CM3410 > SLM
If I were using the Solid Signal DIGIAIR-PRO-ATSC meter, the NF of the 3410 amp would affect the signal quality readings (SNR, BER, and MER) of that meter.