The 12/6 cube paired with the 15/6/1 does an awesome job. I built a 15/6/1 to start, followed by a 12/6 cube, and another 15/6/1. What can I say? These things are a fun project, and you end up with super high end performance.
I contemplated a 15 sub, and decided to go ahead with the full 15/6/1 for a couple reasons. First, the sub alone is not a viable cab. I actually went as far as testing the 3015LF with the low pass filter in a TL-606 cab. I really liked what it did with the 15/6/1, but by itself, the sub was not going to act as a "spare cab" something you might play through by itself. There is nothing going on in the mid range. If you go with a sub you will always carry two cabs. Or you might not feel like dragging two cabs out to play and the sub will sit home.
Now, for slightly more dough than a dedicated sub, you could build a 12/6 cube with the much less expensive Alpha mid, and the simple crossover. Mine quickly became my favorite cab. It covers just about any room we play with or without PA support for the bass. It fits in the trunk of the corolla, I can carry it a fair distance with one hand, and it just plain kicks butt.
I did the reverse baffle so it matches up with the 15/6/1. Paired up this lets my GK 1001 run at four ohms, and I have the high circuit running the horn. The rig stands 5' high. I ask the sound guy to leave me out, because my rig will sound better without the PA mudding the water.
The pair of 15/6/1's have yet to go out together, that's just plain overkill, and to much to carry. I'd say 90% of the time I take the just 12/6 and call it good.
The 15/6/1 are great cabs, but the practical logistics of moving stuff up and down stairs, in and out of the car and clubs, and the volume required at most of the gigs I do, makes the 12/6 my most used cab. If I was to go at it again, I'd build a pair of 12/6 cubes, with the alpha mids. Get a good deal on the 3012LF's and the alpha mids, you might get project done for about the cost of one 15/6/1.
Just a little more refined.
