Guys.....i'm sooooo sorry!!! this problem with my amp was solved (kinda anyway) just a few weeks after my original post (cry for help)... i did write all the info on the matter soon afterwards but, and it's a BIG BUT!!! i must have stuffed up and deleated it instead of listing for you guys to read, again i'm so sorry as you guys helped me loads and it's not right that you didn't get thanked....
         
          anyhoo, so my plexi 50 was a little dumb sounding, almost like my guitars volume was at around 6 when it was cranked...i knew something was wrong!!!! that rich smooth bite was missing and it lacked in REAL TONE like i'm used to....
         
           so, i contacted Nik many times along with help from you guys, i kept trying different tubes (mullards,brimar etc etc all good stuff) and new leads/cables etc etc everything i could think of, and then i tried again and again etc etc for days 'n' days...while still tryin stuff the main fuse blew for some reason unknown to me????maybe a winged c tube had gone faulty? so that was replaced and i loaded in a pair a EH EL34's that where well worn and kept as spares, i didn't want to knacker good tubes....anyhow, i fired her up and WOW!!! something had changed for the better in a big way, there it was, just like that!!! fantastic sounding amp, now, this is where i get lost because i'd tried every tube i had many many many times...the bias was spot on too, about 36mv.... i re-loaded back in the winged c's and they worked great, no problems what so ever, this amps used every day and it's fantastic.......so although the problem went away i don't know and neither does Nik know what the problem was, like i said, i tried all and every tube time and time again, nik did say a valve base maybe at fault? they all looked perfect to me....whatever caused the fuse to blow kinda (in my mind) fixed the fault if that makes sence?        thanks for helping out guys and takeing the time.....anyone thinking of buying a plexi 50 should just buy it!!!! without a doubt!!!! this is my main amp and it's shit hot and as good as most original plexi's i've had/played...

thanks for your time and help............ray.