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,

said above >>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.

said above >>Wouldn't reversing the internal wires to the preamp's output jacks put the signal back to correct phase?

My 2c is that there really is no problem to solve because there is no "correct phase" in the general sense. For a single track you can sometimes determine when the mic and speaker are moving in the same direction and some rigs do come with a switch. However, from one cut to the next there is no standard to set your amp's phase to.

Remember that when you talk about phase you always need to ask in relation to what? Unfortunately the recording process is a long and complicated affair. Phase is inverted and otherwise changed many times. The microphone, cables, mixing console, tape machines and signal processors (delay, echo, reverb, equalizers, limiters etc., etc.) might each be wired out of phase with respect to one another. In addition, smaller phase errors (several degrees here and there) may occur within the electronics of any of these components.

Different cuts from one disk will not have a standard phase and even tracks of one tune will not be recorded so that they have an absolute or correct phase.

In addition, the phase of different parts (frequencies) of the signal will change phase to some degree but not 180deg.
This happens anytime reactance is introduced (inductors,transformers, capacitors etc)in the recording and reproduction of the signal.

Also, in some systems if you reverse the polarity of the speaker wires, and hear a difference, you may be listening to more than "correct phase." Check out this article on SETS and reversing speaker wires.http://usuarios.uninet.com.br/~edelima/REASONS.htm

P.s. the comments in my first post are SET and transformers invert phase too and differential amps work either way...

I remain,

Pops, did you develope the tendonitus loading disks and Lps? The fairly rare and often undiagnosed "music-lover's elbow." Work on loading with the right hand.

I'm in awe!!

Sincerely
I remain