Tube Flavor

Usually the best way is to start with the tubes the designer used to voice the device.,Unfortunately that isn‘t always practicable. In my experience the best way to start is by making sure what you got is performing at its best:
1. use Graphene Contact Enhancer from the Mad Scientist (madscientistaudio.com) on all pins. 
2. use Duende Criatura or EAT tube dampers on all tubes. Not from personal experience but often acclaimed: Herbie‘s tube dampers seem to do a good job as well.
3. start by upgrading the small signal and rectifier tubes with the best NOS tubes you can find.
4. If NOS can‘t be found: in general (but not always): Japanese or German or Dutch tubes are better than Russian tubes which in turn are better than Chinese copies.
5. On output tubes there tend to be pretty decent reviews available on the internet.
Most of all: have fun and enjoy the music!

In my 3 integrated amps and my tube preamp I don’t use tubes from the same manufacturer throughout , use only NOS signal tubes, and where possible prefer to use NOS versus new production power and rectifier tubes.

Below is the link to Brent Jessee’ website that you might find helpful as he provides his insights on various tube brands and the tube equivalents within different tube types. It also lists tubes that he has available which may or may not be of interest to you .

http://www.audiotubes.com/

Likewise, if you enter a particular tube in the AG Forum search function you’ll find user posts. And , below is the link to the Tube-O-Phile Circle on audiocircle which may be of some help.

https://www.audiocircle.com/index.php?board=144.0
Good luck and enjoy

The small signal tubes have the most effect on "flavoring" the sound.The power tubes have a subtler effect and are more about sharper or softer dynamics/leading edge.I haven't tried a lot of NOS power tubes though,I'm a bit uneasy about their reliability. My personal preference is to use new production power tubes and try many different small signal tubes.

NOS power tubes are very expensive, so those are not an option for most people. You would always want to use the same brand of power tubes in all positions, but, depending on brand of gear, you sometimes can substitute a tube or two of another brand in an emergency.

Small signal or preamp tubes can have different functions and different amounts of effect on the sound in different positions in preamps and amps. So you may be wasting the sound of your favorite brand of NOS tube in certain positions in your gear. Less expensive and more readily available tubes may work just as well in those positions.

If you use new production tubes they are generally cheap enough that you can use your favorite in all positions or buy different brands to experiment with without spending too much money.

If you go the NOS route, you will still want to experiment, but you’ll want advice from someone familiar with your specific preamp or amp, as experimenting with NOS tubes can get expensive fast.

If you're asking if preamp and power tubes need to be brand match I would say no. Preamp and other small signal tubes can be a great way to experiment with NOS but as Tom says above NOS power tubes can be prohibitively expensive. What tubed gear are you running I just see a Belles Aria listed in your system?