Many thanks for your replies and comments, it has certainly given me some food for thought. Listening to several preamp shootouts on various forums and reading lots of comments it does seem to be that the differences between pres can often be subtle.
I might well go for one of the Thomann T.Bone valve mics when the budget allows, the clips sound good, they won't break the bank and could be a nice alternative to my CAD M179.
I am still quite keen on getting an external mic pre for my vocals, even if my initial idea of getting something valve based is not the best idea when given my budget. The lack of dedicated pots on the FF400, lack of high pass filter and phase reverse button is proving a little annoying even if the sound quality seems good. Plus it would free up those inputs for some ambient micing.
I guess my question now is, within my budget (£300 ish) second hand or new can I exceed the standard of the FF400 pre's? I have been eyeing up the ISA One in particularly, it seems that this does have a flattering affect on vocals which I certainly like the sound of. There are a few knocking about at £350.
Thanks for the link, is the DBX well regarded? I have put an ebay search up for second hand units.
Many thanks again!