I have an SFL-1 signature and I bought an upgrade kit from Chris Johnson from Parts connection, the upgrade was amazing. If you would like I can send you the parts list and you can check it out. Just send me a private message and I will look for it.
And to answer your question, the sound of this hybrid preamp is a real tube sound, of course it has only one tube. So it is a real tube sound but with some of the detail of the integrated pres, I always felt that its a terrific combination. The one tube is in one side an advantage, because changing only one tube you can tune its sound. I use it with my Conrad MV50 and a pair of Tylo Reference Monitors and I love the sound, warm, great midrange and a lot of extension in highs, the bass is not so good but thats the MV50.
Hope this helps