MIT Cables Z-Stabilizer (original version)

MIT Z-Stabilizer Power Conditioner (original version)
The MIT Z-Stabilizer is designed to work in parallel with your system.  It removes differential-mode noise and audio frequency distortion from the power line. It also improves Power Factor, allowing the utilization of all the available current from the power line.
I found this to lower the noise floor in my system.  
Here's what Spectral had to say about this product:
"Spectral dealers were elated when the original MIT Z-Stabilizer was introduced. Previously we have been unable to recommend any available line conditioner or isolation product on the market due to the clear sonic aberrations apparent when used in the Spectral system. Richard Marsh's innovative parallel circuitry in the Z-Stabilizer solved all previous sonic concerns allowing the first optimum match with Spectral's high resolution power amps. The Z-Stabilizer works in concert with Spectral amplifiers to reject differential mode noise and RFI as well as improving mains Power Factor. No one who has evaluated the combination of the Z-Stabilizer allows the unique purity of Spectral's high-resolution amplification to be fully appreciated."

It should benefit systems using other brands of amplifiers too.