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
Except for a slight bias voltage difference the KT88 & 6550 are completely interchangeable.I haven’t tried 6550’s but I have spent the last few months trying different KT88’s in my Cayin I/A & found the Gold Lion reissues to offer the best balance overall with Mullard reissues slightly better through the mids & actually like the EH in the bass but not so much everywhere else..YMMV,good luck.
I have VTL amps.  I use Gold Lion KT-88's and in the past I've used 6550.   The Gold Lion K-88's perform the best with the speakers I currently use.  My other speakers prefer 6550's.  So, for me it is a matter what speakers I'm using.  I recommend Gold Lion KT-88, great bass and very lush.   I have owned high powered VTL amps for almost 20 year and I'm surprised VTL is not picking up the phone.  Did you buy the amp new or used?  VTL can be picky about used product.  If the amp was purchased from an authorized dealer, VTL considers you a valued customer but if it was gotten used, then VTL didn't make any money.  I purchased both sets of amps used.  I first were 300 Deluxe Mono Blocks.  Considering what we own now days, the 300 were primitive, so over the years I own them, I had it upgraded a few times.  Spending money with VTL has made me a valued customer.  Note: not all tube sellers are equal so if needed search this site for "Good Tube Dealers".
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
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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 people  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?