Tube Amp question:TUBE replacements: explain MATCHED?

Your amp uses 4 kt88s.   You should install 4 of the same brand.  You can buy 4 more of any brand you want.  

Matching refers to tubes within the set of 4.  Since tubes have inherent variability in their responses, people test a large lot and pick out groups of 2 or groups of 4 with very similar responses.   Nice to have but not required.

Now isn't a great time to buy tubes.  But it is good to have a spare.  Your KT88s probably have a log expected life so it isn't urgent to get a spare set.  I would buy a single tube of the same brand you have installed to use as an emergency spare and wait for prices to go down.

Jerry

groups of 4 with very similar responses.   Nice to have but not required.

The Rocket 88 requires (per manual) electrically and brand matched tubes. Some amps require this and some amps don't. 

In the case of your amp, all four KT88's are biased at the same time. If they are not matched for current draw, you may have one or two drawing more current than the others. While the total may be correct,  the individual tubes will be running at different levels. Depending on how far out they are, it could be audible.

The EL84's don't need to be matched, and for equal gain, the 6922's should be matched. That said, I've never seen preamp tubes so far out to make much of a difference unless one was defective.

however I need the plug-in to check the Bias, and I guess a meter?

I just looked at the bias instructions for the 808 and the bias is adjusted by the same procedure as the two Cary amps that I own. A plug (with a + lead and a - lead) insert into a receptacle in the back of the amp; you will need a multimeter that will read DC milliamps. Connect the leads from the bias plug to the leads on the multimeter (the Cary bias plug leads have alligator clips on them) and after the amp is warmed up, you can adjust the bias pot. I always back my bias pot all the way down and start from scratch when installing new output tubes or plugging the amp in at an unfamiliar environment.

Nice that there is bias adjustment. Best case is individual bias adjustment (my CJ amp has this) which means that you don't need a set of matched tubes.