Used Audio Research Preamp Options

 I have also never heard an LS5 in my system. But I believe it is all balanced. ARC sold a converter to use with it called a BL1. It convert single ended into balanced..

The LS models are line stages where the SP models have built in phono stages. The LS3 is solid state. The higher the number the newer the model.

 The back of the  LS25 is the same as the Refs wit a full compliment of both balanced and single ended inputs and outputs.

Preamps have come a long way n the last 20 years. I have owned both an SP8 as well as  an SP9, The LS25 mk2 is much better sounding than any of my previous ARC preamps. When I first got my LS25 mk2 I was amazed at just how much better it was than the original LS25.

Some years ago I was offered a really great deal on an LS27. I took it home for a week to hear in my system. I passed on it because I felt  it was a sideways move. The LS25 mk2 is really a sweet preamp. It is far more detailed with a larger sound stage than any of the earlier models save the Refs. Don't get me wrong , my Ref 5se trounces it. But then the Ref 5se trounces just about everything.
  
If you can stretch a bit you might also want to consider a Ref 2 mk2. Big brother to the LS25 mk2. If you can find one, this is also a really sweet preamp. I could have easily live with this one for life had I not heard the Ref 5se.

  

ARC have a fine pedigree in Pre-Amps and can understand the desire to own one. Tremendously reliable and upgradeable and beautifully designed, most of them have a certain " magic" about them. I still have a massively upgraded SP9Mk11 which has been upgraded way past a Mk111, I stll enjoy. Upgraded the phono is very nice.

But for the money, and as you require line only, I would strongly advise a brand new Vincent SA-T8. You should be able to find one under your budget. Fully balanced tube design, superbly dimensional and musically involving it will draw you into the music like few others. Competes with or surpasses Pre's many times it's cost.

Kalali ...

You asked about further upgrades to your system. Okay, here goes ... Get rid of the Martin Logan Aries and get a speaker that is more efficient and that moves more air. 

I had the ML's for a few years. I changed them for a pair of used Legacy Signature III's.  There are plenty of reviews of the Sig. III's on the net. They love tube amps. The first time I heard them was at a friends home. He was driving them with a modified Dyna 70. The sound was so good that I put my Martin Logans on the market the next day and found a mint condition used pair of Legacy Signature III's on Ebay for $2100. My electronics at the time was an ARC Classic 60 and an ARC SP-14. 

On the preamps ... I've had an ARC SP-6, an SP-14 and I'm currently using an ARC-REF 3.  The ARC SP-14 was in the system for about 10 years. Very nice line stage ... and the one-tube phono section is excellent. 

OP

 You can search multiple sites using hifishark.com

 There is an LS25 mk2 for sale on the AudioCircle Marketplace for $2500.

 There is a Ref 2 mk2 here on Audiogon for  $3,250.00

 Both prices are probably somewhat negotiable. 

Hi Kalali,

To my ears with AR there's a real dividing line over the past 10 years or so and everything that came before. I find the modern AR sound, in SS or Tube, much more enjoyable, fun and natural. Of course, saying that immediately puts my life at risk from those early AR die hards.  The point is,  you should learn which AR sound you like first.

Before buying I'd also listen to PrimaLuna, Conrad Johns on and Cary.