Headphone jack defect in my preamp?

Hello All:
I have owned an Arcam C30 preamplifier for the past several years but decided just recently to listen to my vintage AKG K240 headphones for the first time, believe it or not! The C30 has a headphone jack.
My issue is that I have to crank the volume to the absolute max to get what I believe is still very "low" volume.
My K240's were always great and loud with my vintage Harman/Kardon receivers and I can't believe that they are defective after all these years. Plus, I also tested a budget headphone from Sennheiser and have the same issue.
What's the remedy short of me bringing the preamp. for service? My AKG's and the Sennheiser are the right Ohms for the preamp.
I can't seem to figure this one out!