I'm tube rolling my Yaqin MC13s which has two 12AX7s in V1 & V4 and two 12AU7s in V2 & V3.

So far I've tried Svetlana Winged C 12AX7s and Mullard 12AU7S. I found the Mullards to be really disappointing with a closed in muffled sound.

However, when I put the Svetlana's into V2 & V3 and used the old chinese 12AX7s in V1 & V2 I got a really nice sound, open, smooth and detailed.

I would stick with this but I'm concerned that using a 12AX7 in place of a 12AU7 may harm the amp?

So can I ask if it is OK to keep using Svetlana's in this way. Any concerns?

If not any recommendations for a decent 12AU7?