Subwoofer boom is too much for me...

Try it. 
I’ve done it with different results but IME a properly built sub (ported) should not have a stark difference than a properly built sealed one.

Might try the gain first, and the crossover settings and slopes may have more immediate effect than plugging the port

Agreed

I should have made it clear that only after extensive room treatment (bass traps)

I suppose I have tunnel vision that way…

I would not even consider positioning of sub or subs until after bass traps and/or treatment…

I did do it once that way though…

The room shape was atrocious as well- 15w x 28L x 7.6 high (all plaster walls also bad hard no lossy surfaces) 

only became somewhat acceptable after 4 subs…but the ringing was still generally unacceptable…