OK to use unmatched or different brand tubes?

My preamp has 2x 12AX7 and 2x 12AU7.
I am thinking of trying different tubes.

Would it be OK to use a pair of tubes with the same brand without testing their current levels?
How about using a pair of tubes from different brands with matching current levels?
How about using a pair of tubes from diffenret brands without testing their current levels?

Overall, if a new pair pair of tubes make improved sound, the mathing of current or brand does not matter?
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Power tubes are much more sensitive to matching than pre-amp tubes. But, still it would be nice if they were in the same ball park. A tube tester that can give you a reference number for each is good but not mandatory.

But in general, unmatched pre-amp tubes have not been a problem in my experience.
As long as the tubes are of the same family(IE: ECC83, 12AX7, CV4004, 12AX7A, 12AX7WA, ECC803S, E83CC, CV492, 6681, 6057, M8137, 5751, 12DT7), you can mix and match pairs to your heart's content. You won't hurt anything.
The difference that matched tubes made on early-production Atma-Sphere MA-1s was like night and day. Roger Modjeski of RAM Labs was extremely helpful in this regard.
My original post was based on the thought that you were trying out different small signal tubes to compare them. If you are planning to use them in the system: By all means buy closely matched pairs, of the same manufacturer. Your balance, imaging, etc. will suffer otherwise. They also must be very low noise(tested) for use in a pre-amp.
I will get 50's used Tung Sol and GE 12AX7s. They are individually tested as working, but the seller did not do test on their signal levels and how close they would be matched.
I don't have a tube tester. From reading the responses, at least using both may not do any damage. I'll see how they perform compared to the Chinese tubes.
I wouldn't spend a dime on tubes that were just tested as, "working." Any tube vendor of worth will be able to test for triode(gain) balance within a tube, cathode current, percentage of tube life left, microphonics etc., and should be able to provide you with a closely matched, strong pair(even if they're not NOS). Those tubes may be noisy junk, and could totally(and wrongly) color your perception of NOS tubes.
They are used but tested as like new. So I would expect they are in good shape with their original condition, or close to be labeled as NOS. I spent more than a dime ($8 with shipping), so not a big deal. If they produce less quality sound than the Chinese tubes, I will find another Tunsol or GE 12AX7 and see if they would match better.

Thanks for all your thoughts.
Here's a site with some info on the 12AX7 family: (http://www.audiotubes.com/12ax7.htm) If you like an accurate, transparent sound: Go for a pair of NOS Siemens or Telefunken ECC83 (or better) tubes. If you like a warm, sugar coated midrange: Buy some Brimar or Mullard tubes. Virtually anything NOS(of any quality) will out-perform the chinese tubes, with regard to musical realism.