V1 is the only shared tube so it would be the most likely culprit for a weak tube, but may as well test all the tubes as if they are all barely good it could be a cumulative effect.
Make sure the internal antenna is working by substituting a good external FM ant.
If the tubes are fine check all resistors for tolerance, replace any out of tolerance. Also, check voltages at the tube sockets for any telltale discrepancies and work to correct any found.
If after all that FM is still weak try adjusting the alignment.
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