LOL, thats funny Chad. Glad it sounds so good!...what tubes are in it?
Chad, I had the same experience this last year when I acquired the Aesthetix Calypso pre and Rhea phono. For more than two years now a friend of mine has just been killing me with his tube set up that includes Aesthetix and Atma-Sphere. So like you I had to do it... great to hear your having fun. Enjoy!
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.
Here's a test to clarify this. Plug your CD player into a non-direct input first. Play a CD track that is dynamically/compressed (most pop music will do, e.g., Sugar Ray's version of "Abracadabra" has only 3 dB of dynamic range) and set the volume control to a slightly loud setting and leave it there. Change the CD connection to "direct" input (turn amp and preamp off before doing this, of course, and turn them back on after switching inputs), and play the same track without changing the volume control setting. If the line stage is not used on the direct input, the sound level should drop substantially (by the gain factor of your line stage). If the line stage is active on the direct input, there should be no change in level.