As you will probably get around $2000-2500 for the X1, I assume that is your price range? You didn't specify.
Here are some I have tried recently (running off of a SS amp).
ARC LS26: used $4k, superb, not overly tubey, huge soundstage
Herron VTSP 1a/166: Slightly less detail than the LS26, slightly darker, but still a superb value: used $1800
Dehavilland Ultraverve 2: with good tubes, very musical, hard to beat at the price: used $1500
Maybe others to consider: a Juicy Music Peach II (one is listed right now), C-J 17 LS2, Joule LA-150 MKII, Dodd Battery Preamp. I have heard good things about the Calypso as well. One thing to note: watch the impedance mismatch and input sensitivity with your preamp/amp combo. I had trouble with my McCormack, finding a pre that was quiet in the system, which is why I have been listening to so many recently. Of the 3 listed above, the Herron and ARC had tube hiss audible at the listening position. Only the 6SN7 equipped Dehavilland was quiet. The Calypso mates well with some SS amps (like the Herrons) but not the McCormack, for example.
Here are some I have tried recently (running off of a SS amp).
ARC LS26: used $4k, superb, not overly tubey, huge soundstage
Herron VTSP 1a/166: Slightly less detail than the LS26, slightly darker, but still a superb value: used $1800
Dehavilland Ultraverve 2: with good tubes, very musical, hard to beat at the price: used $1500
Maybe others to consider: a Juicy Music Peach II (one is listed right now), C-J 17 LS2, Joule LA-150 MKII, Dodd Battery Preamp. I have heard good things about the Calypso as well. One thing to note: watch the impedance mismatch and input sensitivity with your preamp/amp combo. I had trouble with my McCormack, finding a pre that was quiet in the system, which is why I have been listening to so many recently. Of the 3 listed above, the Herron and ARC had tube hiss audible at the listening position. Only the 6SN7 equipped Dehavilland was quiet. The Calypso mates well with some SS amps (like the Herrons) but not the McCormack, for example.