I concur with ToeCutter.
Especially in your case, you need two vibration sensors, fore and aft, to
gather info on the engine and prop. You can surmise what the crankshaft
balnce quality is. You cannot do it with one sensor alone unless you move
the sensor to different locations during subsequent runs..

Your post stroungly suggests you need to get find someone of experience to
give you good advice.
I wonder where are you located?

Wish I could help with a good answer but internet makes it difficult.
Answer is only a good as the information provided.

couple of other things:

Sounds as if the operator has a Micorvibe DSS prop balancing rig.
Their manual suggest checking the balnce at 2000 rpm.
Usually I find higher amplitudes of imbalnce and sometimes different phase
at higher rpms.

The previous balance of 13 washers was beyond the FAA approved maximum
correction.

After 600 hours, the isolation mounts can take a set, maybe even have
metal-to metal contact between the bolt and engine mount.
I recall Toecutter suggesting at least a rotation every 500 hours.

Doublecheck the ring gear installation. It will only go on in one position.
And verify the crankshaft index bushing is in the correct location for your
engine.

How are you percieving the resonance at 1500? How did you measure the freq?
have you consider that it might be an idle mixture problem?

good luck

Kent Felkins
Tulsa Ok.