Amplifier Power tubes - important to sound?

i replaced all the small tubes on my mono amplifiers and now I'm left with replacing the stock Power tubes, kt88.

I like what I hear with 12 new small tubes - telefunken nos 12ax7 and 12at7. 

Now I am left with replacing 16 kt88 stock JJ tubes.   will Probably use Siemens 6550 nos.

How much will this impact what I hear do you think?  I recall hearing that power tubes don't make that much of a difference.