This is absolutely the best attenuator I have ever played; not only from the standpoint of transparency, but also from safety as well. With the input impedance matching, your amp "sees" the impedance it expects, unlike an attenuator with a fixed resistor that can potentially throw off the impedance ratios so badly that they damage your amp at high attenuation levels. To complete the SRT technology, the PRX150-Pro also has output impedance matching. This means the speaker also "sees" the impedances it expects, and you'll be assured to get the most optimal performance from your signal at any volume level.

I've reviewed the PRX150-Pro in depth on my blog. You can read about it here:

http://guitargear.org/2009/08/18/prx150-pro-review/

I also encourage you to go to the Aracom products page as well. There are a few great articles that talk about the attenuator and attenuator technology in general.

http://www.aracom-amps.com/products/prx150.html