There was a newer chip used in later versions of the SDP1 which made the unit much quieter. Older units can be sent in to ARC for an upgrade. Also, leaving the unit on at all times makes it noticeably quieter and also better sounding.
The SDP1 is an excellent and non-intrusive piece that can be added to the highest end systems for extracting the out-of-phase ambient information that is contained on good stereo recordings and naturally presenting that information through a pair of rear speakers. An optional center channel can also be connected. The SDP1 does NOT get introduced into the signal path of your front channels in any way and can be engaged and disengaged (returning you to your original two channel system) at the flip of a switch.
I can understand why some who have never heard one properly set up would doubt the need for such a device, but I can tell you that the results are truly amazing. It does require some pretty good cables, amplification, and rear channel speakers in order to avoid degrading the sound of your main front channels.
When properly set up and dialed in, the listener will have no idea that rear speakers are making any sound come from behind them, but will experience a much more three-dimensional front stage, richer harmonics, better decay, and a sense that they are breathing the same air as the performers rather than looking in a performance from the outside.
Very cool product.
