BEST TUBES FOR AUDIO RESEARCH PRE-AMP (LS-16)

I can't answer your question but can offer you some thoughts. I've rolled tubes in a lot of high end gear over the years. While you can generally describe the attributes of a particular tube by type, brand, place of manufacture and year date, to me, much has to do with the overall voicing of the system and what complements it. So, taking a given tube in isolation, no matter the vintage or rarity, will not be a complete answer. I think you have to hear the tubes in situ in your system. 

I never owned the unit you have- I had older ARC stuff, dating back to the SP-3-a-1 and stopped at the SP 10 mk ii (which did use the 6dj8). The problem I found in the SP-10 was that it was highly microphonic-- sort of known for that. It had a sweet phono section. 

I gather that the later ARC preamps eventually switched to the 6h30. While BAT and a few others originally used the old DR Reflector, that tube is now very hard to source, especially early stock. The modern iteration of the 6h30 in my particular line stage- a Veloce Lithio 2.0-- does not have the mojo of the old DR tubes. I did source a matched quad NIB a few years ago that was dated from the late '70s. 

I gather that with more modern ARC gear using the 6h30, the effect of current production (Sovtek or EH) is not as dramatic; that is, the DR variant may not be necessary, but that's a question I'd ask if you are considering a later ARC product. 

I've pretty much stuck with older tubes, though they are pricier than ever and harder to find as true NIB, never used tubes. I think you'll find more alternatives in 6dj8 land than in 6h30 land, especially if you are willing to try older tubes. My only advice is to buy from a reliable supplier. 

I'll let the current ARC tube owners chime in on their experience with the 6h30. 

I think the Gold Lions are smoother (less bright)....which is to my liking...and still detailed...nice midrange...

how much of an upgrade an LS-26 or LS-27 would be.

If its not broke, why fix it?

If you like the Gold Lions, just use them. Not a bad tube at all.
For new old stock, hit up Brent Jesse. NOS can get expensive.

As to the upgrade…the newer ARC stuff is good. It will be quieter than the LS16. It is a cleaner, less tubey sound than the LS16 though, but still retains the ARC musicality and dynamics.

I once had an LS15 that I started with some EH tubes.  I switched to Gold Lion and in my system they made the music less involving.  I switched back the EH and was happy with them as long as I owned that LS15.  I upgraded to an LS27 and never looked back.  I am now on a quest to find a Ref 6 pre. Enjoy your tube swapping. It really is system dependent.