Is your tube amp a "pull" from a console that was wired for Stand Alone?
I have an RCA 6bq5 amplifier that sounds much like yours from your description. The matching preamp from the console was very high output.
Although I haven't gotten around to it, I see no reason why I can't change the input stage and feedback loop to make it louder (more gain).
If your amp is in fact a "pull", you're going to need a schematic with voltages. For example, you can't simply reduce the amount of negative feedback without the risk of causing oscillation or motor boating.
You might want to join one of the DIY forums for more information. That's what those guys do over there, make stuff work.
I have a few different "pulls" and it's not simply adding an on/off switch and mounting jacks. My Fisher 481-A amplifier chassis also powered the preamp and multiplexer before it was removed. To run it as a stand alone, I had to add resistors to make up for the missing preamp/multiplexer. Otherwise, the voltages were too high in the amp.
This can be a great learning experience for you if you have the inclination and desire. I loved my Fisher 481 to death before I got a Fisher SA100, which is what I'm listening to right now. The RCA went to the back of the line because I have six other amps I want to fix first. But I see no reason why I can't make it more sensitive.
What kind of amp are you talking about anyway??