Phase inverted..why?

Generally, the most commonly used tube circuits used to amplify voltage invert phase. That is, the input (voltage swing at tube's grid) and the output (voltage swing at tube's plate) are inverted. This is just the way it works.
Notice at the link below that the sine wave for the output is not only larger because it is amplified but it is moving exactly the opposite of the grid and input voltage sinewaves.
http://www.tpub.com/content/neets/14178/css/14178_37.htm

Typically, you can count the number of such stages and figure out if the amp inverts. Two stages brings it back "in phase" three stages and you are "out of phase." This is not always true as there are some circuits, like cathode follower buffer stages( used to match impedances within the amp) that do not invert phase because the output is not taken from the plate circuit.

Sonically, IMHO, it is not a big deal.

If you are interested read more at the site given above or chapter 1 here:http://files.experimental.ch/Navy/14178.pdf

I think the chapter1 of the above materials is about as good an intro to how tube circuits work as there is on the web.

Cheers
I remain,

Hi Dave, Happy New Year! The usual reason some products present a phase inversion is to eliminate a superfluous stage to restore it back into phase, all part of the KISS philosphy.

I'm somewhat confused and, obviously, not an electrical engineer. However, if the solution to the phase inversion problem is to reverse the speaker cable polarity, why don't the manufacturers just wire the output that way in the amp.

Sorry for the stupid question.