OK, so assuming it's the largest of the Nostromo boxes, which from your description is what appears to be the case, then low output impedance class D amplifiers are exactly the type of amplifier I said in the plans that cabinet is not designed for, which partly explains why you are having some issues, aside from departing from the damping, construction materials &c. Note on the front page where it says:
and
With a class D amplifier it's hovering between maximally flat (rarely a good alignment) and a very shallow EBS. While the amplifier will have plenty of electrical damping, the acoustic alignment is unlikely to suit many spaces unless pulled well out from room boundaries, and having a moderately sharp tuning the effect may be audible. Note this is not the designed alignment, just the result. With a SET or equivalent of about 2.5ohms output impedance (placing in a crucible for averaging purposes), the alignment is as designed, with a slight ~ 1/2dB rise through the upper & midbass, a relatively well-damped rolloff & broader box output through the effectively elevated Q. That was the target alignment and voicing for the design. If you require something different, you need a different design.
