Which Preamp Audio Research LS3B or Sonic Frontiers SFL-1

If you can trade you Nak CA5 for a ARC LS3B straight across I'd do it. If he'll let you take it home to see how it sounds on your system do that first. I had a friend who had a CA5 and I'm pretty sure the LS3 will outperform the CA5. The LS3 was a nice preamp but very revealing so if your system is a little bright or thin the LS3 might not be the right choice. The Naks going to be less extended on top which will appear to be warming sounding. 

Be be curious to hear your opinion if you can compare the two in your system.

i wouldn't discount the SFL-1. With the right NOS Mullard in it will compete with anything in its price range if you buy it right. I owned one so if anyone thinks you can do better I'm curious what it is. Yes... if you can spend $400 or more I'd go back to the ModWright SWL 9.0 SE. Your first two choices are nice preamps if you buy them right. Trading straight across for the ARC is a no brainier in my opinion; at least drag it home to compare.

Cheers

@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.