6th order blowthrough 2 Wolfram AU18s on 8k.

@shredder2 you design blow thru's yet?

That box OP has if it's the old one is for a pair of Tropo 18's on 2K, not designed for a big heavy hi-excursion set of 18's. if the box was left as it was I'd think it would be flexin' like crazy, I'd sure like to see a pic of any mods he did to accommodate the added weight (bracing, extra baffling... etc) and power.

Either way if it's the Tropo 4th I'd sure redesign and rebuild if it was me.

Your sealed size is small IMO.  I haven't ran any numbers but not even 3 cubes per 18 seems small.  Do you know what the Fc/Frc of the sealed chamber is?  What is the port area?

For a 6th, if you want musical I'd do the small end of a ported box for a rear chamber.  Idk what those subs want but you don't want it undersized.  I don't know why people have that perception of undersized rear chambers in bandpasses for music.

I mean bro I did a design for two older x -15's on a sundown 3500 tuned at 43 hz 4th order.  His numbers are really close to yours.  If you want lows you MUST have a proper rear chambers.  I think he peaked in the 150's @ 28 hz  IIRC. 

I just add these videos as proof.  Each ones of these x 15's had 3 cubes of airspace EACH.  He shrunk it later on as he decided he didn't want to focus on lows as much, but my whole point is that you have to do your airspace right, whether 4th or 6th.

It's cause everyone is obsessed with big ratios opposed to what actually works.....8cuft for the rear chamber of 6th order for 2 18s is small....I have a 2 18 6th and my rear chamber is 14cuft Net... does a 156.5 Sealed on the dash and 159.8 on the dash driver door open....don't skimp on the rear chamber volume,you'll regret it...