That sounds a lot more like my own experience building boxes. Getting the pieces to fit well is hard. I wondered why you used so many screws
. But seriously, if you did that well with a handguided jigsaw and maybe a circle saw to cut the panels, you did real well. Maybe you have talent and don't yet know it. There are a lot of tricks to making straight cuts that you can learn as you work. Afterall, MDF is cheap. I also found that a plunge router and a Jasper circle cutting jig helped me quite a lot.
Building smaller boxes for 2-way speakers is actually easier. The dimensions are smaller and if they are for those A/V-1 kits sold by GR Research (is that what you heard?) then you don't need any cross bracing.
The midwoofer in the GR kits, the M-130, is one of those great deals available in DIY speaker building. It costs $24 and is one of the best 5¼" drivers available at any price. It is very smooth across the critical midrange, about 500-2000 Hz, and makes an excellent 2-way when crossed at 2700 Hz to a $30 tweeter. I've heard both the A/V-1 and another DIY version using the same midwoofer with a different tweeter, called the
MB20. I like the tweeter in the MB20 better, but both are very good designs. Some of those speakers along, with your new subwoofer, and you'll be in audio heaven.