Look at it this way...... You have a Center-Taped P-P output transformer... The Center-Tap is essentially AC Ground....

Lets say you use just one tube on only one side of the OPT and not put a tube on the other side of the ouput transformer....neglecting DC core saturation of course...

You have a 8 ohm load termination on secondary, with a 4000 ohm Plate-to-Plate reflected impedance... SInce impedance is turn ratio squared and your only using just half the primary windings...this means that this only tube will see a plate load of 1000 ohms.... which is Raa/4 ....when "looking into" one half of the primary, with respect to ground (Center-Tap)....

Now if you have a true Class B circuit, 2 tubes...that being a tube on each side of the transformer....you still have the same scenario, since each tube acts independent and niether tube is ON at the same time...these 2 tubes alternate, as far as the transformer is concerned, it only sees one tube at a time working, or conducting.... so Class B tubes also see Raa/4 .......

Keep in mind that the two halves of the primary winding, with respect to Center-Tap (ground) are in PARALLEL.....

So in Push-Pull, each tube's plate resistance is coupled in parallel with the other tube's plate resistance as 1:1 ...... In Class B, one tube is ON when the other is OFF, therfore one tube has a plate resistance of infinity when it is OFF, thus not really in parallel..

Class-A push-pull has both tubes, ie.. each side of the primary conducting at all times... In a perfect ideal Class-A P-P situation, there will be no AC current flowing in the Center-Tap, since the net AC current will be 0 , two equal and 180 degree opposite currents will cancel...in real life things are bit different...

The source impedance, will now be considered as the two push-pull tubes in SERIES.... now you don't look at it as 2 tubes...but instead a single signal source of plate resistance x 2 working into the entire Plate-to-Plate load impedance... If you really want to examine what one tube sees in Class P-P, then it's the plate load/4 and the 1:1 coupling of the other plate resistance in parallel, or look at it as rp/2 looking into 1/4 load... That's where composite load-line derivation comes in handy...but after many years of doing this Raa/4 works out close enough.....

Class AB is somewhere between all this, also use Raa/4 .....

You may need to derive these to prove it for yourself...

My advice, don't make an exact science out of load-lines...I did many years ago and wrote many computer programs to do this analysis long before this stuff was popular.... The problem is the tubes you get are SH*T now adays and are all over the map.... and the real life reactive loads vary all over as well....

Find me a .1% tube and a .1% linear load and I will show you a .1% analysis