I have not heard the Line 3se and only had the chance the hear the Line 3 in a dealer system. Although the system had the same speakers as I use, the amps and source were different.
I have heard the BAT line 5i, 30se, 50se in my dealers system and my own. The 50SE is the best preamp I have heard, and I have auditioned quite a few CJ, AR, ML etc. However, I would say that it absolutely must have a perfect system around it, from cables to components to power source to isolation. The problem is that some people buy the 50se thinking it will make the rest of their components sound better....belive me it does not. I think the 5i or the line 3 would be better at this since you can get NOS tubes to message the sound an compensate for other shortfalls in your system. But, if you have a top notch setup (I don't mean in $$$, but in sound) you will not be disappointed...it is that good.