A few days ago I replaced the fan of my Rev. 3 unit with a 'Noiseblocker BlackSilent Pro Fan PE-P - 92mm'. Temperature now is stable at 58°C - 61°C. There is no throttling, average speed is at 820Mh/s. The solution is not perfect as you still can't sleep right next to it, but it is much better than before.
Thanks for that; a quiet fan puts the Single in a totally different light. I've experimented with the following fans on my Rev2 (dual fan) and Rev3 (single fan, heatpipe) Singles and can share some thoughts:
Ixtrema IXP-64-14G (1600rpm, 42cfm, 14dba)
Ixtrema IXP-64-14R (1600rpm, 42cfm, 14dba)
Silenx Effizio EFX-09-15 (1600rpm, 42cfm, 15dba)
Antec TriCool High (2200rpm, 38cfm, 27dba)
Silenx Effizio EFX-09-12 (1300rpm, 32cfm, 12dba)
Vantec Stealth (1750rpm, 28cfm, 20dba)
Antec TriCool Med (1600rpm, 28cfm, 21dba)
Antec TriCool Low (1200rpm, 21cfm, 15dba)
Evercool EGF-9 (1200rpm, 24cfm, 16dba)
If your ambient isn't too high (say, 75F or lower) I've had good luck with the EFX-09-15; it is very quiet and moves a decent amount of air. Not as much as the stock fans (contrary to its specs), but it might be sufficient depending on your ambient temperature. I'll be replacing most/all of my stock fans with these.
The slower/quieter EFX-09-12 doesn't have quite enough airflow to keep the Singles from throttling.
My 2 Ixtrema's don't like being in a horizontal orientation and start to make clicking noises. I'm not sure it is just my 2 particular samples, or if this is a common failing of this model. Otherwise they seem as if they might be a good alternative.
The TriCool is interesting because you can set it to 3 different speeds ('low' is too low; 'medium' might be ok, but there are better fan options on this list at this performance level; 'high' is louder than stock).
The Evercool doesn't have sufficient airflow (even less than the EFX-09-12).
The Vantec Stealth is relatively loud compared to the rest of these but still quieter than the stock fan; it moves more air than the EFX-09-15 contrary to the specs so it may be another reasonable alternative.
I haven't been able to source the Noiseblocker BlackSilent Pro in in my neck of the woods.