New PrimaLuna versus used Audio Research preamp?

You might want to consider an SP-10MKII. Or a Ref 2. Ref2 had tube regulation as well as a 5AR4 for rectification. And 6922, which had the magic missing from 6H30 preamps. IMHO. SP-10MKII is amazing. SP-11MKII is also great,but a hybrid and has a different presentation that is an acquired taste.

Nobody I know can answer how any of these compare to a PrimaLuna without listening in an instantaneous A/B comparison that is level-matched to within .5dB, and double-blind. It’s not possible. If anyone is in L.A. and has a REF 6 or similar they want to bring out, we can host. And who knows. I might be wrong. We did this for some Agon guys years ago with some LAOAC members, and the VSI75. What was shocking was how close good gear is to each other.

If you strip away the obvious prejudice of getting a massive discount from a high, retail price and look at circuit design, quality of parts, and sound I think you would be rather shocked.

only ARC preamp I like is the Ref 10. the LS series is sterile and SS-like. 

I'd try a Primaluna first and roll the tubes. or an LTA MicroZOTL.

“the LS series is sterile and SS-like.”

This is why many make the distinction between the LS and Reference series of ARC line stages. They aren’t one and the same.

I haven't heard the newest Primaluna gear, but my impression of it over a couple of extended listening sessions that it was more "tubey" and very euphonic.  That is what got me interested in tubes actually.

Having said that, I own a LS26 and a Vsi55 ARC integrated, and have heard Tomic601's 5se.  

If you have the funds for the 5se, jump on it.  It's likely to be an end-game piece, and if it's not, you'll recover most of what you spent for it if you're buying used, while even heavily discounted, the PL will lose a lot of it's value if you try to flip it, which you will probably more likely to do.

Some other options to consider.  The Modwright preamps are really nice.  I owned a LS100 for a long time.  I replaced it with a Herron Audio VTSP-360 which I highly recommend.