I have a QR-6500 and recently picked up a QSD-2 to use with it. The QR has the early Sansui QS system for quad decoding and "synthesized" quad from two channel stereo. The QSD has the last version of Sansui Vario-Matrix that enables the use of that feature with any four channel (or two 2 channel) receiver. I thought the QR synthesized quad sounded pretty good. I would notice instruments/sounds coming preferentially from specific rear speakers. In fact, the synthesized quad sound from the QR was noticeably better than the "simulated" quad sound my Pioneer QX-949 puts out. However, many have sung the praises of Sansui's final incarnation of quad QS decoding and synthesized quad from stereo. So, I really wanted to try this and compare to the sound of QR synthesized quad. The best way to do this is to add a QSD-2 (or QSD-1) to a QR system, as it is easy to switch back and forth on one system with the only variable being QR or Vario-matrix enhanced sound. So far I've compared some classic rock (Steve Winwood, Bob Dylan, Van Morrison), and classical music using both systems. Thought I'd just share my opinions of what I am hearing, so this is completely subjective. All tests were performed with the selector set to "synthesized surround." 1. Quality of simulated quad is dependent on the music. Some songs sounded better using the QR system and others using QSD. 2. Separation is noticeably better with QSD; however, this is not always a better "total" sound. I found that sometimes the better separation of the QSD caused a "thinner" total sound. There is less "spill over" of sounds with QSD, but that means those sounds are less present on the other channels. Remember, this is "simulated" quad, not actual 4 channel. With QR (as many have said in other posts) the separation is more "subtle", thus more bleed over of sounds to other channels. To my ear, this often gives a "fuller" surround of sound than the QSD. I think this is completely a personal preference of what you like/what sounds better to you. 3. On some songs there appears to be noticeably more reverb in the channels with QSD than QR. This made me feel like I was sitting in a long narrow concert hall rather than being surrounded by the sound. Switching to QR was better for these songs. On other songs this was not an issue. 4. I have multichannel SACD/DVD-A player and was able to compare the same album (Bob Dylan, Blood on the Tracks), in 5.1 channel and stereo through synthesized quad. Some have claimed in various posts around the web that simulated quad can be as good as true 4 channel, such that buying 4 channel sources is unnecessary. I did not find this to be true (at least for this album). Vocals are much more distinct and clearly coming from the front channels with a 4 channel source. The same is true for other sounds/instruments, and a good multi-channel mix (where the engineer has made good choices) is far superior to simulated quad. Most quad aficionados may know this, but again, I've read some comments claiming otherwise. On the other hand, I only compared one album, so my experience is severely limited. Bottom line is, I am glad to have the versatility of the QSD-2 added to my QR-6500 so that I can choose which version of synthesized quad sounds best to me. I'll have to see which one I prefer in the long run. Right now the QR synthesized quad has a slight edge overall for my liking of synthesized quad. The QSD-2 also adds SQ decoding to my QR-6500, so that is a nice bonus. However, I am glad I did not spend a lot of $$ to get the Vario-Matrix experience. The QR-6500 (and QR-4500) is a very solid performer and well built. Might pick up a QRX=x500 some day to compare the early Vario-Matrix sound. Is it a compromise between QR and full Vario-Matrix? I welcome your thoughts and opinions!