Tubes do go bad over time. That's just something that's going to happen reguardless. The amp would defanately sound much better with some decent volume to it, but that's how tube amps are. If I were you, I would take the amp to a good amp tech, and have them go through the amp to make sure everything is working properly, and do a full retube. This will probably cost around +/-$150(USD) depending on the brand of tubes, and what the tech charges. In the long run, it will be worth it. When practicing at home where the volume doesn't need to be loud, just use the Metal Zone for your distortion.

I'm not trying to tell you what to do, just telling you what I would do. You just can't beat a tube amp, and I've heard a JCM600 1x12 combo and thought it sounded wonderful. Each to their own though. ;)