Tubes for VTL S200

I don't think you made a mistake. The GL KT88s are normally pretty robust. Each brand has a different sonic character though. All the VTL amps run the tubes pretty hard. You may want to buy a few spare KT88s in case you get a failure or two. 

You may want to measure the B+ on your amp. Would guess its somewhere between 550 and 580VDC.  

If you buy tubes from VTL they pre-screen them at the higher working voltages on their burn in fixture to weed out weaker tubes. If you don't want to pay for this, then you may want to order a few spares form normal tube dealers to have laying around. 

Stick with the Russian KT88s or 6550s and avoid all the Chinese versions unless you like tube fireworks. 

Thank you! 

I bought them from Sweetwater but haven’t gotten them yet. I like the S200 very much but feel that with other tubes it can sound even better.

Also looking for black plates. Should all 4 be same model and brand? Or two and two?

NP.

Have not purchased tubes from Sweetwater so no idea on what type of testing they may or may not do. 

As for input tubes you may want to look for either UOS or NOS Raytheon, RCA or sylvania black plate 12AT7s or 12BH7s (on the current model of S200). Depending on which version of S200 you have the first tube will likely be a 12AT7 but the 2nd tube can vary. 

The tubes don't have to be the same brand. 

When buying older tubes just be sure to clean the pins. Any pin oxidation can lead to noise. 

If you are still using the factory installed input tubes, they were usually JJ. They are ok but not the greatest. 

Thank you.

Yes I am still using those factory tubes. The second is 12BH7.