VTL S-200 Tube recommendations

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.