This is just me, but I like to try to keep it around 24-26 m/s. My first ported box I built was showing 28.xx m/s in WinISD, but had just a tad bit of noise around 55hz. You couldn't hear it unless you were right by the box. I rounded the port entrance, and pulled the sub to round the other end the best I could. It fixed the noise issue until I bought a bigger amp. Ever since then I've tried to keep it in the lower mid to 20's. I'd try to make the port a little wider if possible, I think adding 1/8-1/4" more width to the port would drop you in a good range. Shouldn't add too much more length to get to your tuning. If you can afford losing a little, I'd definitely try to squeeze a little more. Hell I'm running 31.5 in^2(18.2 in^2 per cube) for 2 XD8's and half the power you're running. That's just my 2 cents, I'm still learning so I could be wrong.
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