Audio Research REF 3

Dear all ARC REF 3 owners,
I just bought an REF 3 preamp which has replaced my LS25MKII preamp. To be honest i'm little disappointed by the result so far, but i guess tubes have to be replaced. My concern is especially on the sound image which is not precise as my LS25MKII image was.
The number of hours of used displayed by the my REF 3 is 1670 but i realized that hours counter increased only for full hours of used, so i can estimate the number of hours of the tubes inside my REF 3 to be the double i guess.
My question is the following
What do you recommend for tubes replacement and source for the replacement of REF 3 tubes.
I used sovtek, EH gold, and 6H30-DR on my LS25MKII and i was pleased with all those tubes even if i didn't find the DR much better than the 2 other brands.
Thanks in advance
ARCL60