Unmatched power output tubes. Bad idea ?

Most single ended amps, if bias can be adjusted independently, it is not a problem. Quads can be more sensitive since they operate in pairs.

I had a Decware ZMA that had absolutely the best bias system I've ever seen that could balance pretty much any 4 tubes you threw into it.  

Jerry

An interesting perspective from one of the few geniuses in the sport:

https://www.ramtubes.com/article-5

IMHO, it’s not always practical with vintage tubes, and with my modest gear I am really unconcerned and do use vintage Mullard, and Amperex that are not perfectly pair matched. But I prefer new production tubes - maybe that's why. We do the best that we can, things are pretty imperfect at times. Good luck and enjoy.

The VAC is a push-pull amp and will likely work best with matched tubes. That being said, you can try the Mullards especially if they look similar and are from a single source, and bias successfully. 

I see. Thank you. The tubes would be bought from Brent of Audio Tubes, I bought tubes from him before. He himself was not particularly enthusiastic of selling those used unmatched Mullards to me and suggested I contact him in a few months when he might have new and matched ones. So, I will wait for now and consider the options later.

Matched tubes are always better, IMHO.