ARC LS 16 MK1 vs. ARC LS25 MKll

No I haven't, I'm not sure where to start. I am a little intimidated because I have read that you can use a different but, but the unit needs it's bias adjusted.

Is this true?

The REF One does have trimmers. The LS don't need to be biased, only "balanced" tubes are required : triode1 = triode2 for each tube.
You only need to change the ones of the first and/or second stage(s) to tune the sound.

Try the Amperex 6DJ8, still cheap ;-), but sounds good. The current Ei 6DJ8 is not bad also.
ARC units are "very sensitive" to tube rolling, you'll be able to tune the sound with your MkI, KEEP it !!! don't upgrade.

Check on Tube Asylum for more feedbacks and contact ARC for more infos, their staff is very helpful.
I talked several times with Leonard, he's cool.

Your LS16 has only 4 tubes.
The first step could be changing V1 and V2 with a pair of Bugle Boy.
Happy listening !
Emile

Check here, there's a rave review on SP9 + EH 6922 combination :
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1082498473&read&3&4&
I'm going to try ;-) !