Retube ARC VT-100 Mk. II - 6550's or KT88's?

Audio Research tells me that they didn't like the sound of KT-88's when they tried them in a VT-100. However, they were using older Teslas.

Anyone used both 6550's and KT88's, and can give me a sonic comparison?
I heard the opposite when I talked to a sales rep at AR. He said they used 6550's because they were very close, and much cheaper. I have tried both. the kt88's are a little better. The midrange is a little more pronounced, and the amp is a little warmer. The amp sounds great with either tube set.
I agree with John 1's response with the following additions. KT-88s have not always been as reliable as 6550s, therefore ARC has made the choice not to use them in production (as well as for the cost factor).

My experience with the tubes are: I prefer KT-88 for music like jazz and vocals, but for pop / rock, I still prefer the 6550s (NOS).

Be sure that you have a supply of the 1 ohm resistors if you do the DIY method... Learned my lesson the hard way with the installation of Ei KT-90s when I blew a resistor in the amp (which can happen with any tube change). KT-90s are also fabulous tubes...