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

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.

I am running my PH3SE into my LS2B Mk II. For now, I have a matched set and a single (total of 3) Amperex 7308 white label (circa 1965) in my PH3, and a Mullard 10M 6DJ8 in my LS2. If you are interested in finding out more about my system, you can see it listed under FrankC on Audio Asylum's member systems listing.

For cheaper alternatives, some other ARC LS25 and PH3 SE owners have recommended the Russian 6H23N EB for the PH3 SE and Amperex Orange Globe 6DJ8 for the LS25.

The Amperex Orange Globe has a Orange color Globe logo on the tube. There are two types. The pre-1970 design has a ring inside, while the later version has an "A" frame. See JoeS' excellent post on tubes of this family on the Audio Asylum site.