The only way I know how to mitigate losses over purchased tubes would be to do what everyone else does. Try them out and if you don't like them, you just have to sell them at a loss. Just explain in your ads that they didn't have the synergy you were looking for in your system.
Also, as was mentioned above the Electro-Harmonix 6CA7 are surprisingly good. I had a set in my Cary integrated and they were the best of all my tubes, some of which cost considerably more than the EH's. I'll probably get another set for my other Cary amp, as well.