I installed a Zap 2 and a XO2 into a Rotel 991 CD player. The improvement wasn't dramatic but it did enhance the musicality of this fine player. Using balance connections I noticed better extension and definition in the bass, mids seem to open up a bit more and I gained a slight refinement in the highs. The Zap 2 does generate a bit of heat. I recommend it particularly if you require balanced outputs.