I haven't tried any other variants for the input tube regulator and the manual states that the OC2 should not be substituted.

If you are looking to change the sound of the amp, I have found input and output tubes to make a significant difference. There is a lot of information on this forum about the options and differences. One of the easiest to make is to compare 12au7 inputs to 12at7 inputs. I enjoy both, but there is quite a difference between them, not just in gain. That is part of it, but there is some tonal and density difference as well.

If you haven't read through the backlog of MT forum posts, I would recommend it as there is a lot of good information. I hope you're enjoying the amp. I love mine.

I am currently using Ken-Rad 12au7 pewter plate inputs and Phico 6BG6G black plate outputs (which require an adapter). The combination is incredible. These output tubes are relatively inexpensive and provide another tonal option from 6V6s. I've also enjoyed 6F6s which do not require an adapter. Have fun with the options, but be aware, they are many and can be addicting.

I swapped out the OA3s for OD3s only because I wanted the cool purple color.  Not real change in sound