You should have both drivers in parallel for 4 ohms if they are the original 8 ohm EV15L's.
Check out the cabinet wiring to ensure nothing's been fiddled with.
If I remember my old blue line correctly its switched like the old fender bassmans in that the 4 ohm tap is arranged by plugging in only one jack and if you use both sockets, it switches to use the 2 ohm transformer tap.
But of corse the fender is actually an 8 ohm tap and a 4 ohm instead.
My British tube amps all have selector switches and offer two parallel connected sockets. By far the most civilised method as those switchable
Jack sockets can be a bit dodgy old chap!
And we all know what dodgy connections in the output loading of tube amps causes don't we.
The technician species becoming richer while the bass player species gets poorer, often by the exact value of a set of tubes, an output transformer and some labour.
Check everything lest the technicians inherit the earth.... Oh they did already!