Diamond audio’s D6 was one of my favorite sounding subs I’ve ever owned. Not really sure if it’s technically considered an “sq” sub, but it fit in a small ported box, and to my ears, was loud, accurate, and had basically zero mechanical noise through the port. I also ran a focal polyglass sub for a time that was also an excellent sounding sub. It wasn’t super loud, but it was very accurate and blended in with my front speakers very well. It was an 11 inch sub if I remember correctly. This was a decade ago, so a lot of the guys here will have more recent experience with the newer sub models. Right now, I’m on a Dayton audio kick, and I’m super impressed with those as well at their price. I’ve had a dayton audio ultimax 8 on my shop table for a couple of weeks, and in the free air of my shop, it is impressing me immensely. I have it in a 4th order bandpass box. I hesitate to give a full review of it until I have it in my vehicle, as the sound will change in vehicle as opposed to free air, but that little 8 is Impressive so far. I will also be running the Dayton DSP in my vehicle, with an iPad as my head unit, assuming the Bluetooth dongle on the DSP works like I want. im excited to try out the DSP. It will be my first venture into digital signal processing, but reading what alot of guys here say in other threads, it’s totally worth the extra work of tuning it.