Output tubes on a Marshall JCM900

[Netlist]

I'd like your opinion on the tubes I'm planning to change on this Marshall JCM900 100W Hi Gain Dual reverb amp.
http://be.farnell.com/jsp/search2/browse.jsp?N=0&Ntk=gensearch_001&Ntt=6l6g&Ntx=&_requestid=241829

Are these Chelmer valves of any decent quality or should I look elsewhere?

Thanks

/Hugo