I have an ARC LS 25 MK 1.........

I would recommend keeping your MKI and instead upgrade the stock 6922 with other NOS tubes such as the 6H23s (6922) available from Upscale Audio, et al.

I made the same choice. I kept my MKI. I feel strongly, that the MKI with NOS is markedly more musical than the MKII version.

It is probably blasphemy for some, but ARC never did anything for me. You apparently know and like ARC stuff. If you would like to venture outside ARC world, i would suggest Lamm LP2 Deluxe phono-stage! I assume your amp(s)is/are ARC?

You will get a lot of opinions on whether to switch your preamp to something else or not. I think it is all system dependent. You did not mention what amp you are using, by the way.

I will only comment on whether to upgrade or not. The cost of upgrading to Mk II is not cheap. If you don't know much about tubes, and don't want to waste time chasing down quality NOS tubes, your best bet is to spend the money and upgrade to Mk II. NOS tubes are getting more expensive and hard to find, compare to just few years ago. The general consensus is that Mk I with a good set of NOS tubes like Siemens or Amperex will sound better than the Mk II. If you want to try different tubes, check out www.audioasylum.com's tube seller list. They have a good list of reputable sellers. But if you do decide to sell your Mk I, let me know. I've been trying to chase down one for a while.