Warming up the ARC LS2B

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.

I have had the LS2B, and liked it. It sang best with a Mullard. Also, listen to a Amperex BB or PQ. They are nice if you get a good one.

Audio Research hybrid preamps need about 3 days to start sounding their best. If you thought your preamp would warm up in 30 minutes because it has a tube you are mistaken. I have known many audiophiles who never knew this, but thought their system was sounding better by the end of the evening.

The tube in these hybrid preamps has a very long life and if you leave your preamp on 24/7 you will only need to replace the tube once per year. This is a small price to pay for the improvement in sound.

If you don't leave it on you will never know what you preamp is capable of.

Do this before changing tubes. You may be satisfied with the sound.