Audio Research

So, I guess the question is what pre-amp will do better at a tad over 1k...

there were actually two versions of the LS-15 though never a mk2 designation...the easiest way to tell is the second version had a removeable power cord, in addition to better caps and an improved power supply...ARC used to offer the upgrade (75%) of it...

the second version was a nice, low cost upgrade...I had mine upgraded by ARC then moved to the LS25mkll which I liked a lot better...

Rather than buy an old used Audio Research, why not try something like the Schiit Freya for $699?  It is balanced, tubed and if it is as over achieving as the DACs, should sound extremely good. 

http://www.schiit.com/products/freya

 The Audio Research LS25 mk2 is a real jem. I had one for a number of years and was only convinced to upgrade when I heard the Ref 5se. The LS25 mk2 holds its own really well against most current preamps. That is were I would go. The LS15 is a bit dated at this point.