Your diagram is correct for strapping in parallel noob.
So then just to understand. The amp you have is rectified by a set of bridged diodes. Those are solid small state devices and are inside the chassis.
The SE34I.2 opted to use only one set of power tubes and since the "octal" sockets for the inner power tubes are the same sockets as tube rectifiers of several types and the newer amp is running with tube rectification. So no it is not just a swapping of anything.
Noob we definitely got to get you fed with enough info to be "dangerous". Because some of the assumptions your are making could be really dangerous!!!

A set of rectifiers in the power tube circuit. HMMM Lord only know the fireworks that could ensue from that mistake.

So this "hums and screech" thing. That's not right. Unhook everything from the amp but the speakers and put the volume at 12o'clock turn on the amp and the standby switches. SO NOTHING on the inputs, speakers connected, posts strapped red to red black to black.
Any humming and screeching? Something is wrong. Next step is to remove the small middle tube and turn the amp on again. Did it remove the bad sound? If not then remove the left and right small tubes. Any change? If it still acts up then maybe a bad power tube (the EL34's) have extras swap them in one at a time and see what happens. If none of that works then you need to have Decware give it a check up.
Didn't the guy who sold it to you play it for you? Did he give you any tips on operation of any quirks he experienced with it? I turn mine on and it works perfect. So should yours. If it is a grounding issue with another piece of equipment that is another story. I hate searching for ground problems. You'll need the advice of someone with a little more patience and knowledge in that area.