VTL S-200 Tube recommendations

I'm asking our Audiogon's community some advice regarding the power tubes in my VTL S-200. I've tried to contact VTL several times in the past few months about their recommendations, but no success. I understand everything is very slow or even on pause as it is here, due to this COVID-19 disease, but in the meantime it allows us to enjoy listening to music a little more often than usual (I'm trying to see it in a more positive way). The S-200 was delivered new with Electro-Harmonix 6550 tubes, but I'm confused because VTL themselves are stating that it is a KT-88 based amp. Just to let you know that it is one of the most musicaly satisfying amplifier I've ever heard and I'm truly grateful to own one, but audiophilia has got me so I'm wondering which tubes would be the ultimate upgrade? I hope everyone will stay healthy physically and mentally and get back to normal quickly. Thank you for your help.