upgrade from Sonic Frontiers Line 1 to 3 SE+ ?

I would say absolutely yes.The line 3 is concidered one of the best tubed preamps,and with the upgrades to SE+ it's even better.But one thing to take in to concideration ,you could have three line 1 for the price of line 3 se+.

Thanks for everyone's helpful suggestion.
Can I say upgrading from line 3 to SE+ also
has obvious sonic improvement?

Yes, I've had a Line 3 SE for many years, it's phenomenal and IMHO a bargain for what it delivers. It does most everything extremely well: accuracy, soundstage, frequency extremes including a lucid midrange... it's also neutral almost to a fault so you can add your own favorite dash of flavor with tube choices, I/C's, speaker cables, and to a lesser degree: power cables as well. It's an overachiever when originally new and a steal at today's used market prices (plus it can still be fully serviced if/as ever required). Good luck.

Disclosure - I own an Line 3 SE!

After having had many preamps in my system, the Line 3 has become my reference preamp. For the eagle eyed you will see I currently have an ARC Ref3 up for sale ..... I think that says it. IMHO you will never need an upgrade and Chris Johnson at PCX is an invaluable resource should one need tech support. Chris maintains that the Line 3 has circuit design IP that has never been repeated - who knows, but it is both faithful and musical to the source and equally important very compatible with both SS and tube power amp loadings via the impedance curve - not always the case with other preamps if you are mix & match.

The SE+ upgrade plus the replacement volume chip / power supply is the ultimate upgrade to reach world class reference performance.

Thanks for everyone's sharing of experience and suggestions.
I think I will move to line 3 SE then.