When you use an attenuator for WSPR, please use 10 dB or 20 dB. With a step of 3 dB, you will not notice any difference at all with WSPR.
A 10 dB attenuator will not affect reception, just about any commercial radio is sensitive enough. (According G4ZFQ)
WSPR over a long, long time on the same band.
Take the time, to see the absorbing layer, below the reflective layer, disappear.
Even if the M in MEPT would mean, to put your set under your pillow. Hi.
If you stay long enough on one band, you have the chance to notice unexpected propagation.