When I explored packs a while back, LowePro seemed to have the edge in the area of build quality, and at that time, Tamrac didn't appear to offer anything comparable. But since, Tamrac has changed, so the differences now are less critical, I think. So, while I use a LowePro Trekker Classic for my 4x5 or 8x10, depending, I find the design less than optimal from a camera-gear perspective.
If I were to do the whole thing now, I'd follow Graeme's lead, and modify a good pack from a good outdoor shop like REI here in the states. A little Velcro here, a little heavy-duty zipper there, and I'd have one that really works.