A few years ago I sold a conditioner that actually had good-quality parts but, performance-wise, compressed dynamics. The buyer later told me he added bypass caps, and that improved the sound quality a lot.
I've thought about this, and I'm guessing there are some power conditioners that have a good basic design that could be greatly improved with better caps and resistors. I'd be looking at something like a low end Monster Power unit, or maybe even an old Chang unit since those appear to have a good amount of space inside the chassis.
Thoughts?