Tube Amp question:TUBE replacements: explain MATCHED?

I don’t get the mystery behind it all.  I have a Cary CAD- 808…( I’m buying early before I need to replace)… so far, I have bought SOVTEK, in the EL 84, ( new)… I have bought 12AX7WC SOVTEK TESTED STRONG MATCHED PAIR (difference = 0%) TUBES=ECC83, and now I’m looking at (4) Lightly Used SOVTEK KT88’s… I…but I’d rather buy NEW KT88’s… does MATCHED mean I cannot buy another brand/make of a KT88? I believe this would fulfill needs to properly replace what I’m running now…I bought the amp with tubes installed… I am a novice… amp sounds great… however I need the plug-in to check the Bias, and I guess a meter? 

Does matched mean I have to stay with the same brand? I need (4) more KT88 tubes. Do they need to be (4) matched tubes?

SOVTEK only?

Thanks 

Dave