I went to a little yarn shop in Surrey called Knitopia. They have a far larger selection of top quality yarns than the ones in my local area. While there I picked up two balls of yarn to experiment and swatch with. The tweedy grey is an alpaca/wool blend. It feels nice enough but the recommende gauge is 20 sts over 4" and by the time I achieve the required 18 sts over 4" for the Rogue I find the resulting fabric too floppy and limp to effectively hold the cables.
I also picked up "Naturally Merino Fine" a luscious pure New Zealand merino. I love, love, love this yarn! I have only begun swatching with it, but at the recommended 20 sts to the inch it is fabulous! So lovely and soft, yet each stitch really pops. I think at that gauge it would be wonderful for cables. I have to achieve the 18 sts per inch though and I am wondering if, like the grey tweed, it will lose its oompf.
Another problem I encountered with the "Naturally Merino Fine" is finding enough balls in stock of an appropriate color. They don't have any tweeds or heathers of course, and the only grey is quite dark.....more of a charcoal. I am concerned that after all the work of doing the cables they won't show up. This wool is SO soft and lovely to work with that I think I am willing to make somewhat of a color compromise. Perhaps a navy (again, might be too dark). The green I purchased to try it out is one of the colors the store doesn't have in sufficient quantities. I would just order it in, but the saleslady said that they have trouble ordering from that particular company and it could be here next week.....or not for six months of more. I wonder if ordering through the internet would have the same results?