Inverting phase with Joule Electra LA150MKii

I have a Joule Electra LA150Mkii preamplifier. Do I need to invert phase with this tube preamplifier. If so, do I simply reverse the speaker leads at the amplifier outputs?
As far as I recall Joule preamps always inverted phase (I owned an LA-100 MkIII), so changing the speaker leads at the amp output would technically place things in phase again. However, try it and see if it makes a difference. There's been lots of talk about phase inversion with equipment, recordings, etc. Some can readily hear the differences, other have to strain a bit to hear the difference, and still some others don't notice a difference at all.
check phase using the Stereophile Test CD or similar. If you need to invert phase, you can reverse the speaker leads at either the amp or speaker end - just not both :?)

As the others have suggested you can switch your speaker leads. Most people cannot readily hear a difference, and that may be in part a difference in sensitivity to absolute phase of individual listeners and differences in sytem composition, and differences in the recording itself. I have a Levinson No. 32 linestage that allows for phase inversion by remote control. On my system, the differences with most recordings is minimal, and where there are differences, such might be equivocal (e.g.,in one phase setting the vocalist sounds more present and realistic, but the piano sounds phasey, and in the other setting the piano sounds better, but vocals suffer). For the most part, I really do not bother with phase. When the Levinson is used in my friend's system, the differences are much more obvious. I've loaned the Levinson to this person. We both prefer a tube linestage that he has for overall sound, but, he says he often switches back to the Levinson, primarily because of the ease of phase reversal.

As for preferred phase in this friend's system, it varies from recording to recording. So, unless one has an easy way to make a comparison (remote controlled switching) and an easy way to make the switch, I think it is not worth worrying about absolute phase.

Consider too, that some sources invert phase and some amps invert phase, it is not easy to determine what is the absolute phase of the overall system (it may vary by source). So, I would do as the others suggest and switch the speaker leads just to see if, overall, one way works better than the other for you. Don't be surprised if there is no apparent difference.
Many recordings have inverted phase e.g. early Sony SACD's
Its useful to establish, first of all, if your ears are sensitive to absolute phase. So you have to experiment with switching speakers leads.

If you not sensitive then you are lucky. I have Spectron amp with two separate phase switches - one per channel and I can easy hear that some recordings (or acoutis of my room - and I don;t care which is which) is changed and I can easily bring everything to focus by using one of Spectron phase switches.

Its important issue

Good Luck
Good points Rafael, room phase is another piece of the puzzle to deal with.