You might not want to buy NOS tubes until you check the bypass/direct input signal path. Some preamps with a bypass/direct input act as passive controllers in that mode, bypassing all active amplification circuitry. If the SFL-1 does this, you won't need any tubes at all, because they won't be operating in the circuit. OTOH, if it just bypasses some of the signal selection circuitry, it may still use the gain stage. As to why it might sound so much better, you might just be using a "straight wire with attenuation" rather than active gain.
Just trying to make sure you're getting your money's worth.
Just trying to make sure you're getting your money's worth.