In my Cayin A88T I found the SED KT88s to have a somewhat loose, lively, and dynamic more accurate but non-clinical sound. Great bass, good midrange substance, and extended true to life highs with a strong 'front row center' dynamic.
In comparison, I found the SED 6550s in the same system to be a little more euphonic and textured in the central midrange, but slightly less forward than the KT88s in the upper mids. The SED 6550s produced a softer more textured and euphonic midrange than the KT88s, but gave up a little bit of the brassyness in the upper midrange which gives some female singers with an etched voice like Diana Krall and Rickie Lee Jones their distintive voice along with some unavoidable sibilants at the same same. However, although the sound of the 6550s was just a little softer and richer than the sound would likely would in real life, the overall presentation was a little more relaxed and easier to listen to and I noted a distint reduction in sibilants. The tonal shift in sound from the KT88 to a 6550 was a movement toward a slightly less open, and softer mid-hall euphonic sound, but by no means dull. It would be sort of like shifting a little from the sound of a Cary amplifier toward the warmer sound of a older Conrad Johnson amp, although not all the way.
For my system, I ended up preferring the KT88s for its more open vibrant 'front row center' presentation, although both tubes bring something to the table. If I was looking too soften up the sound a bit from a slightly too lively system or listened to classical music or wanted more tonal feeling in the mids for romantic listening, I would go for the SED 6550s, otherwise, if I wanted a more dynamic open sound with good midrange substance, great bass, more accurate tonal representation, and vibrant open top end (without sounding clinical highs like the Genalex Lions, Tung Sol, or EH KT88s), I would go with the SED KT88s.
As a side note, I tried the Genalex Lion KTT88, Tung-Sol KT88s, and some EH KT88s. Both of these tubes have a more linear accurate, yet extended clinical sound to me which many others with good ears prefer. I give the edge to the Genalex Lions over the EH and Tung-Sols in that genre of tube sound, but overall I personally prefer the SED tubes as to me they have more 'body' in the midrange, and although still extended also, they sound more vibrant front row center and less mid hall clinical to me.