Warming up the ARC LS2B

I agree with Jax. The Mullards are still one of my favorite tubes. I also like the Amperex Bugle Boys.

Put some brass Edensound Bearpaw footers underneath..More relaxed and warmer with better bass impact...

Change out the diodes to Hexfreds or Schottkys.
Change the coupling caps to something that has oil like Jensens.
If you are driving one one power amp of input impedance 100kOhms or higher, disable the FET follower stage.
The LS2B is not known for warmth and musicality in the first place.
rolling tubes only introduce marginal changes on a hybrid preamp.
you can put in a lot of $$ buying those pinch waist Amperex PQs and what you get is your imagination tell you that its warmer.

Doesn't someone make a tubed buffer "black box" that is inserted
between the source and preamp?

Google yielded:

Pacific Valve Yaqin CD-1 ($200)
and the ZBOX from Decware ($449).

This might help.

I had an LS2B and several other ARC preamps from this era.
If you don't like its sound (I didn't), look for an SP8, SP10, CJ preamp, or Quicksilver...

The worst sound I ever got from my ARC CL60 was with the LS2.