Rumbling sound with a deluxe reverb head and a 2x12 cabinet

[72Blazer]

My Deluxe Reverb Clone (in a head cab) does not play nicely with a closed back 2 x 12 cabinet.  The output Transformer is 8 Ohms.  I never had this issue when this chassis was mounted in a 1 x 12 cab.

Problem is excessive rumbling.  If I remove the back cover of the speaker cabinet-the rumbling goes away.  I just tried three other amps (heads) one marshall and my super reverb and I don’t have the rumbling issue.  Both are set at 8 ohm selection.

This cabinet has 2 x 12 w/4 ohm speakers wired in series for 8 Ohms (7.8 to be exact).

Question:  Why would the 22 watt deluxe reverb have this issue and the 50 watt marshall and 40 watt super reverb not have the same issue?

Is driving two speakers with the 22 watt deluxe reverb too much?

[72Blazer]

Gents,

I now think the problem is the amp after plugging it into another 8 ohm cab.  Sorry for any wasted brain bites.  I think I will start with rechecking the Bias.  I've had this amp along time.  I did not build or tweak this amp in any way so the issue is new.  The low end gets real distorted past 3 or so on the volume.  Nice sounding below that.