I've owned a Simaudio Moon I-5 for a year now and also own both ProAc Tablette 50 Signatures and Sonus Faber Concertinos. I would guess that the Celeste and Moon amps sound very close.
The SFs have a nice organic sound when mated to the Simaudio. Very full and rich, but somewhat unfocused. I have to agree with Whknopp0713 and admit that the ProAcs are really my favorites with the I-5. The ProAcs excel at imparting a precise and huge soundstage. While some may prefer the tonality of the SFs when being driven by Simaudio, it's the incredible lifelike experience from the ProAcs that really have me enamored with this combination. The ProAcs are a bit leaner than the Concertinos, but still quite warm.
In my room the I-5 is a bit on the lean side so I've had the best results when using copper wire like Cardas Golden Cross interconnect and Analysis Plus Oval 9 speaker cable. That combination helps to give a much fuller sound without any harshness.
For $1000 used you could easily get ProAc Tablette 50 or 2000 Sigs, or even the pre-HOME Sonus Faber Concertos. And BTW, I've heard more than one person say the Soliloquys are a good match with Simaudio too.
Good luck and happy hunting!