Unfortunately this particular batch of M6s have very tight-fitting front/back panels. I drybuilt your pair on Tuesday and was able to slide the front/back panels on without too much hassle - try putting the panel on at an angle, with one of the corners in first, and then rotate it - see my appalling MSpaint here:
If it's still too tight you can always shave/scrape some of the wood away from the conflicting surface on the front/back panels, using a chisel or whatever.
I generally fluff up the acoustic stuffing slightly and spread it evenly over the back wall, it's basically there just to damp down standing waves and so on.
You can leave the crossover at the bottom, there is enough space for it to fit even with the port in place. I usually attach it to the back wall with hot glue, though, as I get annoyed by the rattling sound it makes if it's loose. You could also use small screws, but be careful of going through the back wall panel.
Under the island, middle of a mountain / There is a big bad boomin' system
Blowin' speakers, woofers and tweeters / Amplifiers, melted wires
