Used HFT500 shooting low

First, I would verify velocity with 8.4 and 10.3 gr pellets. Those should border between 11 and 12 fpe. If that's ok, then a drooper mount may be in order. My rifle doesn't need a drooper mount and I have it zeroed in at 25 yards using an Aztec scope. You say the scope as you received it was centered. I don't know what kind of scope it is, but that may be the issue. Do you have enough elevation in it to bring it to zero?
 
8.4 at 830 is a little hot for the 12 ft lb class so it is shooting properly. You can tune that down a little bit with the adjustment screw. As for the hitting low, there are numerous set up that have that result but normally associated with springers. You can shim under the rear of the scope to bring up the POI. I usually use old 35mm film negatives but you can use anything that doesn't smash down too far and is firm. Loosen up the top screws on the scope rings and slide 1 or 2 small pieces of 35mm film as shims under the scope on the rear ring. Works great!
Thx
Dan