Replacement tubes for VTL5.5 preamp?

Has anyone changed the stock tubes in their 5.5 preamp?
All replies appreciated. Mark.
Yes, I have replaced the stock tubes in my vtl 5.5,with (2) G.E. 12AX7,and (4) RCA 12AT7 all tubes were NOS triple mica,black plate,the sound was much better,old tubes sounded, almost harsh with less detail and air.heard alot about 5751 tubes being a good replacement for the 12AX7,tried a set of them and have loved them ever since.But beware,VTL is not happy about replacing tubes other than what they recommend.