That is my horn eBay. I've had it since 1990 and it's been my go-to tenor. The SX is not a model you see a lot. For years I've asked all the pro players and collectors and people that know horns, about the SX and I've only had blank stares. Repairmen and players seem to fall in love with it but no one could even verify that it really existed. Then I found a letter that accompanied an SX that someone was selling for Peter Ponzol. According to Peter Ponzol, it was manufactured in a relatively small batch of 9000 horns, from soprano to bass and it was the best Keilwerth model he ever dealt with. I take the ,"best Keilwerth model ever" with a grain of salt since he was trying to sell the horn but I finally had some kind of I.D. on where the SX stood in the Keilwerth line.
No rolled tone-holes. No adjustable palm keys. Same big bell and sound. It is a heavy tenor. According to Peter, the metal was different (he never said what the difference was) than what was used for the SX90 and as far as playing comparison, he said "you might as well call the SX, the SX80." For me, this horn has played incredibly well and if it sells, I will miss it.