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
ALAN
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
ALAN