A close look at the tubes convinced me to substitute a set of Sovtek 300Bs for the Chinese "stock" models, and a set of four matched RFT 2AU7s for the input/drive tubes. Once correctly re-biased, the tube substitution moved up the performance of this little firecracker. What you get here, especially with the substituted tubes, is a 300B amp that doesn't sound much like those you've heard before.