I am the proud owner of new walnut ULS-15s in the Quad Drive package and I thought I would share what led me up to the purchase and my initial impressions of the sub system.
For about 6 months or so I have contemplated a sub upgrade. For the past 11 years, a Velodyne FSR-18 was my HT sub. While its LF extension was fine, it was somewhat limited in output. This time I wanted something that could keep up with my Klipsch Heritage setup - 4 K'horns (one in each room corner) a Belle center and a pair of Heresy IIIs for the side surrounds. Truly, the new sub would have to have the proper mix of sound quality and output capability to satisfy me this time.
At first, I considered adding another Velodyne but the cost now floored me. For a while I looked around for a used one, but backed away from that idea. Then I looked at the Epik Conquest, but it was discontinued before I decided to make the purchase. I looked at the offerings from SVS and ED, but wondered if ported would sound OK since I had a sealed sub for 11 years. Then I looked at the Submersive - certainly it was a worthy contender. But, Mark said two were needed to really get the most out of it. I did not know anyone locally that had any of these, so anything that I bought would be unseen and unheard and based on other's reviews.
Then I happened across the AVS thread about 2 ULS-15s being equal to a single Submersive. Well, to be honest, I had not checked Hsu out and did not even know they made a 15"er now. Hmmm, looks nice - walnut veneer might look really nice with my walnut Klipsch. Folks are saying that SQ and LF extension both were outstanding. Smaller and lighter enclosure than anything I had been looking at...
I posted a couple of questions about the ULS-15 on Hsu's website and Pete supplied answers very quickly. I sent him a few PMs, all of which were answered same day. Maybe these guys are serious about customer service...
So, the decision was made. Now, Dual or Quad Drive? The discount on the Quad was almost buy three and get one free. Well, whatever I bought I wanted to truly be wowed to make the $s spent well worthwhile. If I bought the Dual, Hsu offered to sell me two more at the same price as the Quad if I did it within 30 days. There's that customer service thing again. You know, I bet these folks will be easy to contact if I need service on one of them...
After all the rationalization was done, I knew that I would end up with the Quad Drive anyway, so why not start with it. After all, Welti's paper said 4 was the best setup. What more justification does a fella need?

So, on Thursday July 30th I ordered the Quad Drive in walnut from Hsu's website. Got an e-mail from Pete saying they would ship the next day. Sure enough, Friday night 4 tracking numbers came in by e-mail. Three arrived on Wednesday, Aug. 5th. FedEx put #4 on a different route and it came in the following day.
Upon opening them, I found the packaging to be excellent and the walnut finish was stunning. I set up the first 3, set all to the same volume at the main listening chair, ran Audyssey, REW and EQ'ed them with my BFD. I fired them up for a listen. Nice, really really nice. Family watched a Blu Ray movie and I was really impressed - wondered what # 4 would do for me.
I found out the next day. I was absolutely FLOORED at the difference the 4th one made. Same setup, same EQ settings, but magic was happening. A whole new level of performance. They were set up along a 12.5 ft wall, so at least two were within 1/4th wavelength of each other up to crossover, so I was getting co-location gain.
Another poster expressed his feelings about the ULS-15 as "HOLY COW!" Well, after a few high volume pipe organ tracks and a few SACD tracks, my impression was "LORD HAVE MERCY!!!!!"
Family watched "Knowing" on Blu Ray last night. Not only did the Quads perform well at high volume, the LF extension, impact, and overall smoothness was there even at "wife's volume."
Let me close out this long post by saying that the mission is truly accomplished. The Quad Drive has a permanent position in my HT. There are so many sub choices on the market now, but let me assure you that you cannot go wrong with the ULS-15.
I will post a picture of the setup and a REW FR plot in another post.