WGS ET-65 Possible failure

Hi all, what kind of issues would one expect to see for a speaker failing??

I have a TWO DSL100 heads (one new and one old J2k) and both amps go into thermal runaway (right side tubes) due to what seems to be a speaker failure and Im not sure it really is. My normal cab is a SUNN T412

The first time this happened was in my 4x12 cab (two Celestion G12T75 and two ET-65). The ET65s were put in few years back due to the bottom two speakers in my cab getting wet. So ive been getting this failure of the tubes running away (JCM2k) and I thought it was the tubes but the EXACT same condition happens on a completely different amp (DSL100H).

SO I pulled out the ET-65's and put them in to my two 2x12 cab (parallel 8ohm) and the condition STILL happens. After playing with the DSL100H for some time, started hearing some loud pops and noises (independent of volume, amp was really low, clean channel) then the valves started to runaway again.

Ive swapped out the ET65's for the Celestions and the condition has NOT happened yet (the celestions are at least 18years old)

Now, three weeks ago when this initially occurred (when all speakers were in my 4x12 and using a jcm2000 head), I was able to use a different cab (marshall 1960a) and the problem did not occur for the rest of the evening. A week later, with NEW HEAD with the SUNN, the same issue re-occured with the new head (blew a HT fuse) Yet again I tried a different cab (same cable) (Soldano 4x12) and the problem did NOT reoccur for the rest of the evening. Which now leads me to the current situation of the 2 ET-65's in a 2x12 causing runaway again.

Until my G12T-75s exibit the same issue, I cant rule out the speaker cab.