Question about guitar tone

The comment about tube amps breaking down is bullshit. If you have a shitty amp and treat it like shit, it will break down. If you take reasonable care of it, and if it's built by someone who knows what he's doing, it will work just fine.

Replacing tubes is not that expensive. Everyone makes such a big deal out of it and thinks that you have to replace tubes every few months or something. The truth is that the preamp tubes (normally 12AX7's) will last at the very least a couple of years (and that's if you're gigging regularly and really pushing them. The average user may not even need to change them unless they break). The tubes in the power section will have to be replaced more often, but even that's not going to be as often as most people think. As for the price of new tubes, depending on your amp and how many tubes it has, you may spend as much as $200 or more for the really ass kicking tubes. Stock ones aren't all that expensive.

The amp itself is obviously more expensive, but maintenance isn't as bad as most think. If you have the money and need an amp for a loud rock/metal band, do yourself a favor and get a nice tube amp, as nothing in the solid state world can compare (unless you prefer the SS tone). Not only do you get a fatter, warmer tone, but you get balls to the wall loudness. A tube amp will be significantly louder than a solid state with the same wattage. Plus, if you really have to push your amp, the tube one will be able to handle it much better, as tube distortion is far less dangerous than SS distortion/clipping. Clipping your SS or transistor amp can not only damage the amp, but also the cabinet through which it's running. Plus, there's nothing like the tone of tubes being cranked.

Long live tubes.
 
I gigged about 9 nights per month with my tube amp, and never had a single problem with it. It's worthwhile to have an extra set of tubes for a backup, but it's really not that likely unless you bounce the amp around that you're going to be wrecking tubes.

As for the midrange question, the mid knob varies from amp to amp. It has been my experience that Marshall tube amps have a very smooth, musical midrange that can be turned up quite a bit.
Most Peaveys, on the other hand, have a rather harsh midrange that needs a certain amount of cutting. In any case, if you completely negate the midrange, you will sacrifice both a lot of cut as well as a lot of note-to-note clarity.