The LS 100 was my daily driver for a long time and was really good with many different amps. Dan Wright is great to work with. There's a pretty good deal on one right now, no relation with the seller:
When I settled on Atma-Sphere amps (first the M-60s and now the Class Ds) I eventually found an Alma-Sphere MP-3 which, unsurprisingly, has a bit more synergy. The MP-3 is generally more expensive than the LS 100 but I found my MP-3 for less than $5K; they come up occasionally if you're patient. Ralph Karsten is also very supportive.
My spare now is a LTA MicroZotl MZ2S which is surprising close to the MP-3 and can be found in the $1K range. LTA also makes higher end preamps which are less than $5K used and likely better than the MZ2 but I haven't heard them.
There are a LOT of good preamps in your price range.
