What's your "diminishing returns" preamp choice?

If you can find one, the Ayre K-5xeMP is an amazing preamp for the money.  New it was $3500 to $4350 depending in the year but was brilliant.  You should be able to pick up a used unit in the $1800 to $2200 range depending on condition.  

I regret selling mine simply because it was such an amazing unit for the money.  I have added two units to my system since it left (PS Audio BHK and Art Audio Conductor) and they are both better but are quite a bit more expensive and the Ayre deliver 95% of the quality of the PS Audio and 90% of the quality of the Art Audio.  

Ok, so I still gauge diminishing returns against a venerable preamp I bought in the early 80's, the DB Systems 1b/2a/4b.  I've had it updated twice.  Other than a brief stint with tubes, it's my reference preamp.  The only reason I'm not using it right now is it doesn't have some of the features of the newer preamps.  I think I paid something like $800 in 1982 money for it.

I had a Thor TA-1000 MKII preamp that retailed new for around $11K. Having tried a number of different preamps over the years, I genuinely thought that the Thor was at the line of diminishing returns, and that it wasn't really necessary, sensible, or practical, to spend more.
That was until I heard the Allnic L-5000 DHT, which I now own. I didn't think there was THAT much more performance to be had from a tube linestage, but I was mistaken. Yes, the Allnic comes at a much higher price, but it's one of those rare pieces that make me think it was money well spent.
I'm curious to hear an Atma-Sphere pre and also a BAT at some point in the future. Perhaps the line can be nudged a little further still!

would be interesting to have a 'quietest' contest and then a 'musicality' contest between the benchmark and an ayre k5xe mp

Very good recommendations above but some say that the preamp colors the sound. But how do we know that the power amp don't color the sound as well since the signal has to go through different circuits there.