HI Psarkissian, Joeleft,
Thanks for your posts. @psarkissian I will try a last time with a Roland UM one mk2 midi interface to be sure that it's not a midi issue at all, but in the meantime I have contacted Audiotronic in Bordeaux, France. It's the nearest authorized service center (at 350 km from my home). I will keep you posted about this aventure.... I hope it will be ok because i liked the sound of this amp. It's a second hand from a guy that didn't liked it after 6 months, so I hope the warranty will work for me.
@Joeleft yeah, I agree. I saw a lot of posts in this forum regarding midi issue with DT25/50. I think L6 made a bad design. Too bad because linked with the HD500x, it rocks ! If it's the tubes, I saw that Eurotubes sells special kit for DT25/50 with bias indication to do it yourself. It may be worth a try for less that 100$. It could resurrect it ...