Help me choose an Audio Research preamp/line stage (Please?)

You can find the LS-27 for $3500 - very sweet.

I’ve owned LS-15, LS-16, LS-25 and Ref1.

Based on your requirements, I’d go with Ref1, 2 and 3. Also LS-25 (original, not MkII) would fit the bill.
The Ref series preamps sound big and dynamic. The LS-25 is a touch leaner sounding compare to Ref series but very good. I would skip LS-15 and LS-16.

The newer ARC preamps are quieter, have lower noise floor and don’t have the old school ARC tube rush (hiss). They are faster and have higher resolution. You still get the tube magic but the overall sound is more accurate than the old ARC stuff.

OP thank you for this thread.  I am in the same boat, but my budget is $5-6k.  Based on everyone's posts I should go with a Ref 5 or Ref 5se.  Or should I go with a LS28se?

Why does it have to be AR ? 
there are plenty of excellent preamps out there.

go to. Linear Tube Audio 

their Micro Zoe under $5 k is excellent and you have 4 tube choices .

Your problem is your budget will put you in the older units near rebuild time Capacitors especially older units caps start drying out in 10 years loss if fidelity 

like hearing less and less , resistors too far better out there now ,old carbon type warm but gritty vs the new 1% Takman orDale  Vishay for example 

coupling caps , put in VH audio Odam small but very good  find a good tech 

and high offender the crappy old tube sockets , for $25 each you can get far better Teflon gold over Copper ,I experimented just changing the tube sockets a solid 10% increase in fidelity why 3x or better conductivity ,and far less resistance pit was like installing a good vintage tube ,I was shocked this was 7-8 years ago.

everything counts  put it on credit buy new , the LTA micro Zoe would be perfect.

another overlooked element  the volume pot itself you loose a lot of micro detail and imaging using a $25 Alps or Bourne volume pots ,they are just conductive plastic wipers , many older and new preamps use them why $$ to save 

you should just go to Schiit audio and buy the Freya +  put a decent aftermarket power cord like the Pangea sig mk2 -$200

and buy good NOS  tubes 6SN7.  I have this in my backup system with nice tubes 

go to Brent Jesse recordings, or vintage tube services , I don’t know your cables or front end ,each tube sounds different , check it out ,it is very respectable with good new old stock tubes I put 1958 Raytheon tubes in there very nicely balanced 

the best are probably the 1940-50s Tungsol top oval getter, or sylvania silver metal collar base tube , but $$ , hint hint Many companies has their name put on the tube like Hammond organ , Delco ,Fisher ,when in fact rca, or Sylvania and brand new and tested you can even buy a 2 year warranty for $15 a tube , these off names tubes are less then 40% of the correct name brand , . Brent Jesse Carrie’s a lot of these .having been into modding Everything over 20 years , and tubes over 40 years and owned a Audio store and had service techs learned a lot .

just check out the Schiit audio brand $895 and just buy your own tubes.

NOS far better then these Russian  fake Tungsol  they are using the quality ,sonics,and longevity , not in the same league.