Which Preamp Audio Research LS3B or Sonic Frontiers SFL-1

@edenton925 ... just FYI, the original Sunfire 300 is not a fully balanced amplifier. (Years 1996-2000)

If your amp has "300 - two" printed on the lower left faceplate, then it has a balanced design. (Year 2004, aka the Sunfire 300 x 2)
Input sensitivity for rated output
Unbalanced: 1.64 Vrms
Balanced: 0.82 Vrms

http://www.thecarversite.com/manuals/mandir/Sunfire%20300X2%20600X2%20owner%20manual.pdf

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!