Ok, I am getting a little annoyed with these subs. Been playing with the phase and thought I had it perfect. The subs are co-located in the left corner of my room and when I test the M&K, it is pretty smooth down to around 20-22. Add in the second sub and although it sounds more authoritive, it has a few more nulls and peaks. It's not completely waco but it's definitely not as smooth.

I wish someone would get output figures for one HSU ULS-15. It would open my options up regarding placement and hopefully meet or exceed my current output. Lots better WAF too.

Have you placed the subs with an RTA and microphone with sine wave output being sent to both to positively confirm that you have negative phase as this will see it better than an SPL db metre.