I am new to this and replacing my RE SXX 15 worried after a year it sounded cool, but the thing on E40 etc. to long might have to be turned down got a little smell.  I have looked at Xcon wanting something lower and only way I could fit an 18" would be in a sealed box, upgrading to try and get more low end, handle Hifonics Brutus 2400 better, and hit all day.  It will be one speaker guess everyone seems to call Xcon SQ sub more than Zcon,  15 would bolt in box net slightly over 3.55 aero port 6" might upsize box and run 4-5 4" tune it lower even in current setup probably tune it lower then RE was.  I looked at the Zcon 15 because of the better coil and actually sensitivity on sub seems better.  I know they say Xcon are under rated and not sure how much the Hifonics actually puts out but as it only ad on was an extra battery, big 3 and car voltage stays in 13s if not 14s 13s mainly at idle.  I got hooked on larger subs when I had truck and 2 Kicker competitions ported into the back, sealed and sensitivity lower but sound always got compliments on sound descent levels for me on a daily driver.  Tried the 4 12s more ear piercing then sound and lost low end or for myself preferred the sound of the 18s.  Only way I could do a Xcon 18 would be in a sealed box not sure how that would work, too bad they did not have some in shops in area but I never heard one and guess just wanting to buy the right setup.  Plus if we play a bass only disc something that will hit which the SSA line seems to be the way to go.  I am new at this and probably older than most of you doing this so please bear with me and any suggestions are more than welcomed.  I am in a rural area of only 6000 persons so I do not have the options of running to a shop like I did in Sacramento and looking at specs and outputs using box design software.  Hoping and trusting in getting help here.