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
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I thought the VTL was a wonderful amp but I bought as a demo. When I called them with a question Bea was extremely unpleasant. It got to the point where I refused to call them. I've bought other product as demos or used where the company was extremely helpful. They know that the used market for components is large because peopleĀ  want to upgrade equipment. What if they want to try another brand? Bea expects them to keep it in their closet or junk it? Let me say she did help me but wasn't the least bit pleasant about it. The fact that the building is closed is no excuse not to answer or return calls so with the company attitude it figures. As much as I love their product even if I had the money the idea of dealing with her is very distasteful after the way she treated me in the past.
The dealer I bought it from said that VTL was supposed to buy it back from him but did not. If this is true what choice did he have?
If people upgrade their VTL equipment does the company take back the old giving credit for it?