Unless it's a rare output transformerless (OTL) design, your amp has an output transformer between the power tubes and the speaker. This transformer will not pass DC unless it is seriously defective, in which case you'd likely have more severe symptoms such as smoke and overheated output tubes.
If it's not coming from an upstream component, then as mentioned above, low frequency oscillation is the likely culprit.
I agree that it's time for a trip to the shop.
If it's not coming from an upstream component, then as mentioned above, low frequency oscillation is the likely culprit.
I agree that it's time for a trip to the shop.