First Sound compared to Supratek, VAC, or Audio No

Hello. I have a FS Prescence Deluxe II and am very happy with it. I have read that people often prefer transformer coupled preamps to non-transformer coupled ones.

Does anyone have experience with the First Sound products and one or more from Supratek, VAC, or Audio Note? If so, how were they different?

I addition, I am curious if any First Sound owners have moved up their line? Where did you hear improvement? Overall, do you think is it worth the extra expense? Looking back, would you made a different choice? Thanks for your input. Brooks