Which Preamp Audio Research LS3B or Sonic Frontiers SFL-1

Is that "minimal sum" more than a better performing used Herron VTSP-1 cost? 

SF SFL-1 $500 +/- 
SF SFL-1 Signature $700
ARC LS3 $700 +/-
Herron VTSP-1 $1400 +/-

Never listened to the Herron pieces but have read many great things about them. Won't argue its likely the better prramp but it's going to be maybe twice the money I think.

Any of these preamps are going to be a nice step up from the Nak. I think you started with you can get a great deal trading your Nak towards a SF or ARC. If your friend with the ARC is local try it first. I still would take the SF over the ARC but you can't demo it unless the other person is local too. 

Forgot to to mention prior to my SFL-1 I had a ARC LS7. The LS7 never did it for me. The SFL-1 was musical and I enjoyed it. For what I sold it for I should have just kept it.

I auditioned the LS3 against an Audible Illusion 3a, Conrad Johnson Pv2, and a First Sound Passive.  It's all subjective and depends on the system, but I thought the Audible Illusion pumped up the sound too much, the Conrad Johnson was too soft, the First Sound was kinda lifeless, and the LS3 seemed to get things right, delivered clarity, kept the meat on the bones, and got out of the way.  I eventually kept a Pass AlephP, but didn't think it blew away the LS3.

My SFL-1 with a Telefunken 12AT7 was a very neutral, good sounding unit. That's a years ago memory, though. 

The ARC LS-3 I recall, also years ago, was a balanced version of the LS-1 which was the Line Stage of the SP-9. I performed the "accepted" mods to m y LS-1 removing all the white by-pass film caps, replacing the 100uF electrolytic in the power supply, upgrading the output caps to Obbligato oils, and using an Amperex 7308. 
Those mods improved the soundstage, tone and texture but it really came alive when I shoe-horned a TKD volume pot in place of the Alps.
I miss that unit... Damn this hobby!

Kitch29 I think is referring to the LS2 rather than the LS3. The LS3 was a solid state unit as the LS2 was a hybrid design. I'd for sure pick a LS2 over the LS3 but expect to spend a bit more?  Both the standard LS2 and LS3 where singled ended; the B designation behind the model number was the balance version. I believe the balance versions were a $500 option and not sure they were a true balance design but just added XLR connections.

A tube preamp, even a hybrid is going to give you the option to roll tubes in and out to flavor the sound to your liking; whereas the LS3 it is what it is unless you do some cap modding which could be hit and miss and you better know what you're doing... plus it likely will devalue it some. You'd spend more on great caps than it's likely worth. Better just to move up to a better preamp than spend money on a LS3 if you're thinking of modding it.

Again the LS3 is a nice preamp for the money. Sure you could go out and spend a little more for something like a Parasound P5 with a lot more options and full remote control, but I bet the old ARC LS3 is more neutral. I've not compared the two side by side but I've heard the P5 plenty and for the money with all its options it's a very nice unit, but I'd take the old Sonic Frontiers or ARC over the P5 if I only needed a line stage. Only negative with the older units is no remote.

Also remember with a hybrid preamp you'll have the tube maintenance cost which is not bad; nothing like power amps. Some rare NOS tubes can get spendy. Tubes will last a long time with normal use. Just don't leave the unit on and forget about it. The SF SFL-1 only has a single tube so it's affordable to run.