Since you are not experienced with tubes you definitely need expert help. Spend a little time reading on Audio Asylum. They discuss who to order tubes from, and the symptoms of tube decay. Naturally, the manufacturer will be happy to sell you a new set of tubes, but be prepared to pay double for what the same set can be had for from other suppliers. I can recommend two reliable suppliers.
Antique Radio Supply, for tubes in general, especially rare numbers. Jim McShane for expert matching and advice on power amp output tubes. I have had good service from others, but these two have been outstanding.
Antique Radio Supply, for tubes in general, especially rare numbers. Jim McShane for expert matching and advice on power amp output tubes. I have had good service from others, but these two have been outstanding.