How to 'measure' subs

Didn't see your question until now. If you haven't already figured it out, its 2V for 1W in 4ohm.

You just use Ohms law: P = U² / R

So 8ohms- 2.83V @ 50hz, 4 ohms- 2.0 V @50Hz?

Yep

this seem accurate?

did a test with a 118 peavey theatre sub VS york_ville sw_80 8 both loaded with same driver (peavey black widow 1808) that is the driver that the theatre subs come with. Dark green is the 808.

2.83 volts (or damn close) at 50hz at 2 meters.

Nice to see you are getting on with the sweep measurements

There is something that doesn't look quite right. Could you take some pictures of the speaker while it is setup for the measurement with mic placement and all. Preferably from different angles so I can take a look at the surroundings also.

I think the best idea is to practice a bit more on the simple reflex sub.(Peavey)
I found the graph of a newer version of it and it looks quite close. I just want to make sure what dips are real, and what are due to measurement error.

You can see the 2 orange lines spray painted on the concrete. (the 2 meter lines) Closest one being for the sub firing away and furthest line being for the mic facing in.

As you can see below their are some boundaries.

Ah yes both to the left and the right and quite close
There is definitely some cancellation because of that.
How solid is the stuff very close to the right?

Take a look at this:
www.genelec-ht.com/learning-center/faq/acoustical/bass/

In your measurements of the 218 horn and the peavey I can see what looks like at least a few consistent dip at around 30, 50, (90) & 110-120 Hz. How does that correlate with the distances from solid objects?

Its raining here today unfortunately, but I thought I might try (tomorrow) to aim the cab at the corner after covering the small stone wall in carpet that I have here. See if that does anything........

You can always give it a try. I am not sure I understand the aiming part?

It might also be a good idea to try a standard 1 meter measurement. That should give you 6db gain so less impact from the sources of interference hopefully. Maybe even 0.5 meter.

If you are wondering what object that causes the interference. You can try to do a measurement and then move 1-2 meter away from that object and see if the dip moves accordingly.