I think removing Bias is a key.
If you can remove bias from the study, then it does not matter if they are staff members or not.
I don't see bias in Amp DBT since I can't tell the difference.
I think it's okay to use anyone, staff or not, for amp testing.
But I can usually tell the difference in bass responses among speakers - not all of them, but most of them.
Couldn't you pick out the speaker "A" that goes down to 35Hz @ -3dB vs. speaker "B" that can only go down to 50Hz @ -3db?
Even if you prefer the midrange/HF and overall sound of speaker "B", you could still "favor" speaker "A" if you want to every time.
I own the Linkwitz Orion, KEF 201/2, Revel Salon2, & Infinity P362.
In full range mode (Pure Direct 2.0), bias will be present because I could tell the difference in bass response of all of them.
Well, I could tell the difference between the P362s vs. the other 3 no matter what.
But in 2.1 stereo mode, all crossover set to 80Hz, I would have a hard time telling the difference among the KEF, Revel, and Linkwitz speakers-- I think because most of the bias (bass) has been removed.
But a lot of people say they can tell the difference in midrange and HF of all speakers. They say they can pick out their speakers. If this is the case, then bias is always there.
So, in conclusion, I think if we remove the bias from the equation (play in 2.1 stereo mode, crossover to 80Hz, etc.), then it does NOT matter whether they use their own staff.