Pre-amplifier suggestions

Back in July, I asked for SS amp recommendations.  Since then I have acquired some Krell "KAS" monoblocks and some ML 33H.  I have listened to both fed by an ARC Ref 3.  The Krell were more revealing, the ML sounded dark.  This difference prompted me to dig out my Pass Aleph P (pre).  With the Aleph there is an immediate and extremely noticeable improvement across the entire range.  Now, i am wanting to upgrade my preamplification. 

I am considering the following preamps in no particular order:
1. Dartzeel
2.  Esoteric Grandioso C1
3.  Vitus
4.  Solution 520
5.  Boulder 1010
6.  ML 32 or 52
7.  Nagra Classic or HD
8.  ARC Ref 6
9.  Absolare Passion
10.  First sound
11.  Atma MP1
12. Bat Rex

As you can see I'm all over the place...The Krell KAS has balanced inputs,  the ML has both balanced and single-ended inputs.  I use long balanced interconnects, the amps are 16 ft. away.  If you have experience with either of these two amps, please provide a preamp suggestion.  Lastly, I would like to keep this under $15K (preowned).

Luxman c900u is my #1 choice but I will first try the Benchmark LA4 on the 30 day home trial because it may work for 1/6th the price,  the amp is the Luxman m900u. The c900u can hang with the preamps on your list.

I am favoring the LA4 if I keep my amp about 15-20 feet away. It supports the XLR standard, AES48, which may allow me to use regular professional quality XLR instead of the very expensive audiophile cable. The Luxman c900u does not support the AES48 standard  (some other preamps supporting AES48).

I am expecting the LA4 to pass the signal without flavoring the sound. What I should hear is the flavor of the amp. The Luxman c900u should add flavor to the sound compared with the Benchmark.  I say "should" because I have only compared the c900u with the m900u. In the future will compare with the Benchmark LA4 with the Luxman m900u amp before I make any purchase.

An A'gon user recently bought the c900u to pair with his m900u and he told me he tried several  preamps, BHK, Pass Labs, PS Audio DS Direct, and the c900u was the best by far. Which is bad for me since the c900u is $15K. However, the Benchmark at $2.5K is worth a shot to see if it comes close.

With the Aleph there is an immediate and extremely noticeable improvement across the entire range.
Aleph L very hard to get one, it was one of  Nelson's favorite preamps.
Bit of a chameleon, it's passive preamp up till around 3 o'clock on the volume control after which if your system needed it, then turns slowly into active with varying gain up to full volume.

Nelson Pass
Unique to this preamp, patent pending, is a volume level control which combines the best qualities of a passive attenuator and active gain circuitry:
At the 3 o’clock volume control position, the Aleph L offers a direct path from input to output.
The only component in the signal path is wire and switch contacts.
At positions below 3 o’clock, the volume control functions as a precision passive attenuator using discrete resistor ladders.

Above 3 o’clock, active gain is added to the output signal in 2 decibel increments, for a
maximum of 10 dB.
As a result, you suffer the effects of active circuitry only when additional gain is necessary.

Cheers George    
Thank you both for the suggestions.  I have reached out to Benchmark and may try one out.  I am also going to audition an ARC Ref 6.  

I own a single-ended CAT SL-1,  I need to purchase RCA to XLR adapters in order to use it, as I don't have long RCA cables.

Why the Boulder 1010 not the newer 1110? Finding used Boulder equipment is tough (I’ve tried). I get the impression people rarely sell unless they are going up the Boulder line. 
I'm trying to keep costs below $15K (pre-owned).  
I was quite happy with the ARC Ref 3, until it was paired with the ML 33H.  And so it goes on...