I built a pair of M2s lately and I have a few questions. I quickly checked the tuning and it seems to be 24 Hz, not 27. The ports are 8 3/4" or 22.2 cm long with a rounded outlet and the internal volume should be really close to 140l. I used a rigid type of damping material inside, but I know I didn't put in enough. It's probably only 3/4 thick. Would that cause the tuning to be too low? I'm going to play them a bit and check the tuning again.
Is anyone using an Xilica DSP? I am using an XP-2040. I adjusted everything so it was pretty flat according to REW, however it sounded a little strange. I removed some of the input filters and it sounds better to me. Right now they sound as good as my 4345 clones.
Here are some pictures of the flared ports. I don't have access to a CNC to make the lower part of the baffle round, so I just rounded the edges. I didn't want to try to do it by hand.