How Important is Tube Matching?

The best thing to do is consult the product documentation or simply ask the manufacturer. There are varying approaches in the industry as some amps and preamps have autobias features while others do not, not to mention differences in circuit implementation...

Much depends on the design of the unit involved. Lots of tube amps can sound just fine without matched output tubes if they are designed to allow for biasing of each tube individually. Some pre-amps, especially those that operate in a balanced mode absolutely need matched tubes to get the best sound. Some preamp tubes need not be matched at all because the benefits obtained in matching do not affect the sonic output. But, for the best sound I have always sought out matched tubes for critical applications in pre-amps. Unlike power tubes in most pre-amps there is no way you can adjust their output. Besides, the matching process should cull out noisy and microphonic tubes as well. The can be very important.

Don't know anything about 6 channel pre-amps or whether the design of your amp (?) makes matching necessary, optional, or a waste of money. One thing does come to mind though - if you are using one tube for both stereo channels, getting a tube which has been tested to insure that both sides of the tube are matched should benefit the sound.

There are many  design of  using preamp tubes like 12ax7,ECC88,6SN7 and other double triode tubes.If the design is a cascaded type,you don't need a matched tube but if it's in PP phase splitter,you need a closely matched tube.