The most overlooked reason has to do with the fact that you set up the speakers for the bass balance of the other pre amp. Did you adjust the placement of your speakers after instaling the new one? Every time you change something in the system that effects the tonal balance it's time to adjust the position of the speakers. It may only take a 1/2" adjustment. It's free. It's the single biggest reason most of these guys never get anywhere with their systems. They plop in some new thing. "Oh gee", "it's this", or "it's that", up for sale it goes. Did they ever move the speakers? I doubt it.
Save the great tweaks for after you get the basics right. Then those things will just put a smile on your face. You may need to move them again after the new tubes or the new power cord. Give it a try,very small adjustments can make a huge difference.