I respectfully disagree, or would at least say that op amps can have just as a dramatic effect as tubes.
I only have one set of tubes besides the stock ones, and using them sounds "different" than the stock tubes.... but I ordered 3 different op amps from TI (for free) and those were all night and day from each other. And they were each better than the stock op amp in my humble opinion. (Can remember the model numbers off hand. I settled on the one I liked best and never looked back)