I am in the process of setting up my first dedicated home theater and am looking at upgrading my sub (currently a Cerwin Vega ARPH 02300). My room is 21 x 14. My other components are an Onkyo TX NR708 receiver, Artison Portrait LCR (older model that I just bought used), and Artison Portrait LRS (bought new old stock). I am still up in the air about adding two more speakers to make it 7.1 so I have not purchased anything for that channel. My walls and ceiling are drywall and I have porcelain tile over a concrete slab. I have three doors and one window in the room. I keep the doors closed during movies. I can try to sketch something if necessary to show the layout. Based on the above what size sub or subs should I be looking at? If there is some additional information needed please let me know and I can try to provide that as well.