Is it mostly a problem getting the tubers to grow initially? Those that have grown, how have they performed?
You have such great Roses, and I kinda think of Dahlias as having similar needs, plenty of sun, nutrients, compost, moisture, and air circulation. So its kinda perplexing what the problem may be with Dahlias? There are some iffy sources out there selling Dahlias- years ago, before I knew much about them, I ordered some from Jackson and Perkins and received some tubers with no eyes, which of course didn't sprout. That's the only experience like that that I've had, but it makes me think there probably are other sources selling eyeless tubers out there.
Another possibility just came to me. I've noticed some tubers are slower to "wake up" in spring than others (by "wake up", I mean they start developing eyes and sprouts). Tubers that haven't started producing eyes can be more prone to rot upon planting, particularly in cool, moist soil. I usually lay sleeping tubers in a flat in early spring and pack some slightly moist potting soil around them indoors. That will typically wake them up, and I start seeing eyes and sprouts within a couple of weeks, then I plant them out.
