I know John Ulrick, the owner designer of Spectron, and I am sure he would be more than happy to give you an answer. Just email them, they are extraordinarily nice folks. Of course they make their own pre, which is perfectly suited for their amp. That doesn't mean that it is the best sounding available, it only means that it is designed to work well with their amp.
I am told that the First Sound by Emmanuel Go, is a great choice.
I currently have a Belles 21A, (Dave Belles' new model) and it is excellent. It takes a couple of weeks to sound like it can, but that is a wonderfully good sounding preamp for $2500.
I just came in from owning a Gryphon Callisto, and I am not disappointed, even though that is a $7000. Integrated. This tells me that Belles is really doing somethng correctly. The Belles preamp has no real personality of its own that I can detect yet. Certainly no overly ripe mushy tube sound, but just sweet enough to be non transistory sounding.
Also, I highly, HIGHLY, recommend the use of the Spectron Sense cables also. They complete what is tantamount to, for want of a more in depth description, a comparator circuit, for the amp, and therefore an integral part of the amplifier.
If I can help email me.