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.

Rwd:
I am using the VT100 MKII with my LS 25 MKI also. The combo with the NOS is awesome. A set of four 6h23 NOS will cost about $100 ($25 each)