Review on Enhance Your Home Audio Experience with the FX AUDIO Tube Preamp: NE5532 Replaceable Tube and OP Amp Chip, Mini HiFi Stereo 6K4 Vacuum Tube Buffer Preamplifier with Bass and Treble Control, and RCA Cable Compatibility for Amp/Active Speaker by Justin Santiago

It's like a great entry-level tube amp, but it's just a preamp and you can't drive passive headphones through the outputs. This is a passive preamp with stock 6K4 tubes replaced with 5654W tubes that make the amp sound better, with more separation and clarity. Please note that the amp comes with an incredibly short power supply. I had to replace it with a Yamaha Portatone Keyboard power supply that has the same voltage and polarity as the tube amp. Additionally, even with the best cables in the world connected to my Monolith Liquid Spark, you will encounter some background hiss at medium volumes (see images). Maybe I'll try the NFJ&FXAUDIO Power Conditioner Adapter to see if the hiss goes away. Otherwise I have to invest in a better tube amp as an additional tube amp in addition to the SMSL SP100 (see pictures).