I think major factor affecting malt quality is not the maltster's brand name but the quality of barley (and wheat). This depends on the conditions during growth and especially harvest season and depends where the growers are located. All the major malt houses are probably able to buy good quality grain when it is available from their contract grower and they also have the know-how to malt it properly. Slight differences in flavor and color may exist but even these may arise partly because the grains are grown in different places. So more important than the brand is choosing the right product for the beer style (and using correct amounts of it). If something seems to be wrong it would be easier and cheaper for a homebrewer to switch between brands (growing conditions) than for a commercial brewery such as Mikkeller that probably buys huge amounts at a time. And pay attention to how you store the grains and especially don't store crushed grains. You can experiment with different maltsters and see what your personal taste buds prefer. I like the idea that the grain and malt is produced and malted in the same region where the beer style originates. So for British beer I would use British grains malted by British maltsters (Simpsons, Fawcett, Crisp, Warminster, Bairds, Muntons etc.) and for Belgians I would use a Belgian maltster (Dingemans, Castle Malting, Swaen) and so on. Germans maybe Weyermann, Best Malz, Avangard etc. At the same time I need to say that I know many people who do not follow this rule and they brew great beers using local malts only. For example, here in Finland people brew many styles using Viking malts that are available, affordable and the grains are grown in relatively unpolluted northern soils. The results can be great. And if you experiment and try to create something new you are obviously not restricted by the geographical origins of the malt.
