Tubes used in well-designed audio circuitry offer a clarity, richness and purity of tone beyond what is achievable with solid-state (silicon or germanium) transistors, FETs, opamps or even the latest digital modelling technologies. To my ear, transistors and FETs often sound overbright, clinical and even brittle sounding. They are susceptible to transient overload and clip in a harsh and unpleasant way. Digital gear is even more problematic and will splatter the audioband with unrelated harmonic content if signal levels are too high. With careful design these effects can be minimized, however tubes are inherently linear, have immense headroom and produce beautiful soft-clipping when pushed out of their linear region. They’re the right tool for the job of audio signal processing. On an historical note, the first commerically available transistors were used in the mass manufacture of portable radios. The sound quality of these was not exactly audiophile. Additionally, I don’t comosder operational amplifiers appropriate for audio signal processing either. These were originally designed for the purpose of analog computation before digital computers were technically feasible. A typical op-amp contains dozens of transistors and diodes on a silicon substrate and are always used in circuits with negative feedback apllied to control their gain and Bode response.