I never learned transconductance, they didn't teach it at ET 'A' school.
But the 6CB5 has a transconductance of 8800umohs.
The 6L6 has a transconductance of 5300umohs.
And a 7591 has a transconductance of 10,200umohs.
So the 6CB5 has a gain about inbetween a 6L6 and a 7591.
Of course the second section with a gain of 5.4 will be in a cathodyne circuit, so it won't have any gain.
With NFB, the first section's gain might be more like 10 to 20, so total gain will be 10 to 20.
If you do a LTP instead of cathodyne, and use 2 6CB5s you can get plenty of gain; 40 x 10 x 2 = lots. Leave off the cathode bypass capacitor on the first stage and you should be golden. Something like 25 X 10 X 2 = 500 - losses here and there.
A 6CG7 will work well to. It'll give you a gain of 10 X 10 to 20 = total gain of 100 to 200.
Make sure the transformers are on the bottom, so if it tips no one will get squiched.
At first I wanted to be a ET. Now I are one.