Upgrading Sonic Frontiers Line 3SE to ???

Have you tried a different amp to see if that makes a positive difference in your system? I'm using a Line 3SE and don't have issues with dynamics and bass weigth, but I've got Bryston 7B-STs so there's plenty of bass drive.

the 3se is one of the finest pre's ever. the bass issue u are you have is more likely one of the room, the amp, or your speakers. you can pend more on a pre, but most likely you will just get different, and not better.

I did bring my Line 3SE to a friend's house for A/B in the past, his ARC Ref 2 Mk II outgun my Line 3SE on bass weight and dynamic for sure. I later found out the 6GH8 was worn and replaced them, that restored some of the dynamic but 6H30 pre (ARC Ref 2 Mk II or BAT 51SE) do have an edge on dynamic from what I read/heard.

Currently I have Sylvania 6GH8, Siemen 6922, and Sovtek 6H23 in buffer. I have plenty of Valvo and Tunsgram to put in buffer after 6H23 are worn. What will be a good choice for driver without costing a fortune. Don't have $500 to spend on a pair of pinch waist, but can swing $250 for a pair good tubes if that will improves the bass.

I tried many amps in the past, but only BAT 75SE gave me the goosebump I was looking for. For sure 75SE does not have solid state like bass and that's the reason of 2nd amp like Pass, but I like the amp enough I am willing to search for a new pre to go with it.

Amperex A-Frame 6DJ8 in the buffer should help you achieve the dynamics/bass weight you are looking for.

I think you'll get more dynamics and slamm by going with solid state amp with more power than your current BAT amp.
SF speakers typically are laid back sounding, so if you want more slam, pair them with SS amp.
Also, you may want to have your friend bring his ARC preamp to your place to compare. This way you'll know better in your system and your room.