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.
chrfor
Thank you for the input. I bought the amp new from an authorized dealer. I had a technical issue with the first amp and after some discussions with VTL, the distributor and the dealer, VTL is not answering anymore. I've been told that they wouldn't answer the customer in the future.

Wow, sad to hear.  I've only contacted them once since getting 450MKII amps in 09 and that was fuse related.   
Quick update for those interested. I have never received any response from VTL, but decided to give the Gold Lion KT-88 a try. I can't say it is night and day with the stock Electro-Harmonix 6550, but it is a nice improvement to my ears. The instruments have a little more body and texture, and the vocals are somehow a little more forward. The music is very well presented and superbly balanced. I also decided to change the driver tubes also for the Gold Lion. I don't think I will go back.
Tube Depot has them Gold Lion KT88 for $60 each. Are they the same ones y’all talking about?
https://tubedepot.com/products
I may give them a try since mine are  still 6550 ELectro-Harmonic