We've seen Sililoquy speakers set up at the 2000 CES with the Celeste gear. Their website is www.solspeak.com. We didn't get a chance to hear them, but Sim must feel that they match well.
I heard the Sim I-5 driving ProAc Response 1.5 (MIT cables) and thought it was exceptionally good. Sim also likes Totem and Gershman, judging by their own demo's. I think Sim gear (which I own, too) is better with richer, warmer speakers that really benefit from the control of the high current amps and that can handle dynamics well. Vandersteen's might be a good fit. Sonus Faber Concertos are excellent, but I' never heard them with Sim electronics. I would also try any of the Aerial's that fit your budget.
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!
Soliloquy voices their speakers in part with Sim Audio amps(I got to visit the factory), so that would probably be as good as any place to start. I've got the 5.3s(driven by McCormack DNA-0.5 Rev. A) which are excellent--very detailed highs without glare, clear and full mids, plenty of punch in the bass(I had to plug the ports to even them out in my room which is a little smaller than yours and is square), and they disappear in the room and yield a large 3D soundstage. Best of luck.