Have listened to both the early AR Ref one preamp and AR the Ref two.
compared to the VK-51SE i can say with much confidence that the VK51SE is much better in every aspect.
the first thing you will notice is the transparancy which is really in a different category.
The VK-51SE is like a passive device with gain.
loads of gain actually.
back to the AR Ref series. compared to the VK-51SE they sound rougher and have congested highs. to me it is this obvious.
I have had the experience that all AR amplifiers have this 'curtain over the speaker' thing.
have had an VT-200 mk2 hooked up to my Gen 501's for a week, same experience like the Ref series pre-amps. compared to my Atma-Sphere MA-2's its like losing 5 Khz in upper extension and air.
the midrange / lower midrange has a better texture though and has more bloom. i like that aspect better with the AR VT-200. but i go out of topic here..
If you want an edge of your seat experience; buy the VT-51SE if you can.
other pre-amps compared to;
LAMM
CAT SL-1
Cello Encore
Cello Audio-Suite premium
AR Ref
others